Enduro means riding or racing dirt bikes over challenging long-distance terrain.
Enduro derives its name from “endurance” because the races are typically long and span difficult and varying types of terrain which usually involves riding on an off-road path or track, but that track is usually not easily passed. There may be creeks, rocks, mountains to climb, or other obstacles along the way. Enduro used to refer more to a cross-country dirt bike race, but now is pretty much used any time someone is riding on rough off-road paths. “Enduro” differs from “Trials” in that a normal dirt bike is used instead of a trials bike and the terrain is longer but generally less difficult, but so-called “hard enduro” blurs the lines between enduro and trails dirt biking.