Snorri gives the bridge a load limit and then tells you when it will be exceeded.
Gylfaginning: the bridge has three colours, and it is very strong, and made with more skill and craft than other structures — and even so it will break when Múspell's sons ride over it. The red band in it is burning fire, and it is there because otherwise the frost-giants and the mountain-giants would go up into heaven.
A bridge is a defensive liability. Anything you can walk over, someone else can walk over. So the Norse solution is a bridge with a lane of fire in it, and the fire is described not as decoration but as the reason giants cannot use it.
Bifröst or Bilröst?
The manuscripts disagree, and the two forms mean different things.
Bifröst contains bifa, to shimmer, shake or tremble — "the shimmering way" or "the trembling road." Bilröst, the form in the Codex Regius text of Grímnismál, contains bil, a moment, an interval, or a weakness — giving "the fleetingly-glimpsed way," or "the road that will fail."
Neither is obviously secondary. Röst is a road or a distance. The second element is stable; the first is not, and editors choose. The name in Grímnismál's stanza of superlatives is Bilröst: best of bridges.
Snorri also calls it Ásbrú, the Æsir's bridge, which is the least interesting and clearest of its names.
Is it the rainbow?
Snorri says plainly that men call it the rainbow, and that is the reading everyone has inherited. Three colours, an arc, a bridge between earth and sky.
There is a competing proposal, made repeatedly since the nineteenth century, that the original image was the Milky Way — a permanent band across the sky rather than an intermittent one, more plausible as a road, and with parallels in other traditions where the galaxy is a path for souls or gods.
The argument for the rainbow: Snorri says so, and the three colours and the arc fit. The argument against: a rainbow is not there most of the time, and a bridge the gods ride over daily ought to be. The point is not decidable, and it is a good example of how much of the tidy modern picture rests on one thirteenth-century gloss.
Who cannot use it
Grímnismál stanza 29 lists the rivers Thor must wade every day to reach the gods' assembly at the well — Körmt and Örmt and the two Kerlaugar — while the others ride. Snorri explains why: Thor does not use the bridge, because the fire would burn it if he rode over it, and he wades instead.
Norse mythology's strongest god cannot cross its most famous structure. Whether the reason is his weight, his heat, or a taboo the sources no longer preserve, the exemption is stated as a fact of daily practice, and it means the picture of Thor thundering across a rainbow — which is more or less obligatory in modern illustration — inverts what the text says.
Heimdall lives at the bridge's end at Himinbjörg and guards it, which is his entire job description.
The breaking
At the end, Múspell's sons ride out under Surtr, and Fáfnismál names the island Óskópnir where the gods and Surtr will meet, and says Bilröst will break as they cross and their horses will swim the river.
The bridge does not fall in battle. It fails structurally, under a load, exactly as Snorri said it would. That is a very unusual note for an apocalypse to strike — the world does not end because the fire-giants defeat the defence, but partly because the infrastructure was never rated for them.
What it is really doing
Bifröst is the only permanent connection between Ásgarðr and Miðgarðr in Snorri's cosmology, and it is guarded, restricted, burning, and known in advance to be temporary. Every attribute is about controlling access.
Compare that with how the giants travel: on foot, over land, arriving at the wall or at a hall. The bridge is not how the worlds are connected in practice. It is how the gods commute to court.
The bridge Marvel built
Marvel's Bifrost — a beam of light that teleports people, operated by Heimdall with a sword in a mechanism — is now the dominant image, and its destruction and rebuilding is a plot point across several films. It has almost nothing to do with the sources except the guard and the name.
The word is in ordinary Scandinavian use for bridges, ferries and companies, and there is a real Bifrost in Iceland — Bifröst, a village and university in Borgarfjörður. See world trees and the axis of the world for how other traditions connect their levels, and the Norse overview.
If Bifröst burns giants, how do giants keep getting into Ásgarðr?
They walk. The builder who wants to wall Ásgarðr arrives on the ground with a horse; Þjazi flies; Þrymr's messengers travel overland; Skaði comes in armour to the gate. Nothing in the corpus shows a giant even attempting the bridge. That is why the fire is presented as a preventative rather than as something ever tested — and it suggests that the bridge belongs to a vertical cosmology (heaven above, earth below) that sits awkwardly on top of a horizontal one (Ásgarðr and Jötunheimr as neighbouring territories). Both are in the sources, and they are not compatible.
Does anything cross Bifröst besides the gods?
The dead do not — Hermóðr rides down and north to Hel's realm, not over the bridge, and the einherjar arrive by unspecified means. Snorri's account has the Æsir ride over it daily to their judgement-seat and back, and that is essentially the only traffic described. The one crossing that matters in the other direction is the last one, when Múspell's sons break it. For a structure so central to the modern picture of Norse cosmology, its recorded usage is remarkably limited: a commute and a collapse.