The Narrows in Zion isn't a trail. It's a slot canyon you have to walk up the river to enter. The Virgin River carved the upper canyon into a passage where the sandstone walls close down to twenty feet apart and rise a thousand feet on either side, and the only way through it is to wade. The Riverside Walk leads from the Temple of Sinawava shuttle stop to the river, and from there your route is upstream in the water itself.
What "bottom-up" means
There are two ways to do the Narrows: bottom-up (start at the Temple of Sinawava, walk upstream as far as you want, turn around, walk back) or top-down (start at Chamberlain's Ranch outside the park, walk 16 miles down the canyon over one or two days). Bottom-up is the day hike — no permit required, no shuttle complications, you can turn around whenever you've had enough. Top-down requires a wilderness permit and serious commitment. Most visitors do bottom-up.
How far you can go
The legal turnaround for bottom-up day hikes is Big Spring, 4.7 miles upstream. Most parties don't get that far — they make it to Wall Street (the narrowest section, about 2 miles up) or to the Orderville Canyon confluence (also around 2 miles), and turn around when the wading has worn them out. The trip is round-trip, so whatever you walked up, you have to walk back down. Time-wise, plan on three to seven hours depending on how far you go and how strong the current is.
Wading is the workout
You spend most of the hike in moving water — anywhere from ankle-deep to chest-deep depending on water levels and which channel you pick. The river bottom is rounded river-cobble that's slick under any boot. Most experienced Narrows hikers rent a canyoneering package from one of the Springdale outfitters: neoprene socks, canyoneering shoes (sticky rubber, drainage holes), a wading staff. Outfitters in Springdale — Zion Adventure Company is the largest — rent these packages for around $30/day in season. You can hike the Narrows in regular shoes if you must, but you'll be cold, slow, and prone to falling.
Flash flood reality
The Narrows is a slot canyon and the entire upper Virgin River drainage funnels through it. Flash floods have killed people here. The NPS issues a daily flash flood probability rating at the visitor center and online; "probable" or "expected" ratings close the canyon outright, and "considerable" ratings get rangers strongly discouraging the hike. Storms thirty miles upstream in unrelated weather can deliver pulses to the lower canyon. Read the day's rating before you go and trust it.
Water temperature and season
Late spring through early fall is the comfortable window — water temperatures in the upper 50s to low 60s F, air temperatures warm enough that wet hiking is pleasant rather than dangerous. Spring runoff (May through early June) brings high flows that close the canyon when the river is over 150 cfs at the Temple of Sinawava. Late fall and winter, the river is cold enough that hypothermia is a real concern; most outfitters add drysuits to their rental packages in those months and the NPS rates the route differently.
Wall Street and Orderville Canyon
The two most photographed sections of the Narrows are Wall Street (the narrowest stretch, where the canyon walls are at their tightest and tallest) and the Orderville Canyon side-canyon, which enters from the right about 2 miles upstream. Most parties poke a hundred yards into Orderville to see the slot opening and turn back. Wall Street is the section that delivers the headline Narrows experience.
Where it fits
The Narrows is one of two Zion trails — alongside Angels Landing — that show up in every "best hikes in America" list. Unlike Angels Landing, it doesn't require a permit lottery; unlike Angels Landing, it requires you to commit to being wet for hours. The two are the headline experiences in the canyon and most first-time Zion visitors do both. Returning visitors often skip Angels Landing and do the Narrows again — the canyon experience is the kind of thing that rewards repeat visits at different water levels and seasons.