A pipe-like or box-like construction of native rock, wood, metal, plastic, or concrete under a trail.
It catches surface water coming from side ditches and directs it away from the trail.
There is generally a catch basin above the trail and the culvert, buried under the trail, carries water from the catch basin to discharge it on a lower hillside.
Sometimes rock sheathing is laid on this downhill side to slow the flow of water and prevent erosion there.
You know what a culvert is. It's a short sewer. Thingie.