A waterbar (also called an 'interceptor dyke' according to Wikipedia, though that's referring to use on roads) is a trail feature used for erosion control.
The difference between a hiking trail and a creek bed may be apparent to most of us, but water doesn't quite see it. An ideal trail will be outsloped so water flows right off the side, and crowned so, well, ditto. Too often, though, trails are built going up an incline, and old trails tend to become anti-crowned (concave)--basically, becoming a riverbed.
The solution is to put a diagonal channel across the trail, diverting water off the side. This can be done using a log, rocks, or just dirt, though the latter tends to wash away.
A waterbar should be at roughly a 45 degree angle across the trail with the high end away from where the outslope would naturally be.
There's also a preference for putting them at the bottom of switchbacks, as the water already wants to go off the trail rather than making a sharp turn.
The downhill side should not be below the bottom of the log/rock(s), as it will eventually wash out water will flow unhindered below your bar. Useless.
The drainage of a waterbar will tend to get filled with debris, so they should be punched out from time to time. If they're like witches (made of wood), you will eventually need to replace them, as wood rots, albeit slowly.
One caveat: waterbars should probably be avoided if your trail is used by bikers. They are essentially angled steps.
Sometimes the trail is just way too anti-crowned and de-berming them so the water can go off the side would be a massive pain. Maybe the berm is solid rock. In these cases, we tend to do check dams or check steps to slow the water and force it to (hopefully) deposit its sediment.
It's probably not typically seen as an alternative solution, but on the South West Rim trail in Big Bend National Park, we actually built a causeway--a raised section of trail with a drainage running along side it. Normally I believe one would reserve such constructs for marshy areas.
On wheelchair accessible trails, one should just go with a rolling grade dip. Just barely noticable bar across your trail covering--crushed granite in all I've built. If your trail is ADA-compliant, it's pretty flat anyway (5% grade), so you shouldn't have water running streaming down it anyway.