Sliding can refer to low-profile forward-moving attacks. Slides are powerful because of their gap-closing potential and lengthy active windows. Not only do they advance a fighter forward, but they also prevent enemies from approaching at the same time.
Long, far-moving active periods on slides have interesting applications in terms of frame advantage and potential combos. Advantage is dependent on when the slide hits; hitting with the tail end of a slide results in way more advantage than hitting earlier. Some fighters can take advantage of this with a guaranteed follow-up combo, particularly if the slide has a launching effect.
Sliding also refers to momentum that carries a fighter across a stage briefly, perhaps in a state where they shouldn’t usually be moving. The biggest example is reverse-rushing—running, then pivoting to slide backturned. In older Smash titles, sliding right after air-dodging into the ground enables some movement tricks, but the potential for post-air dodge sliding is greatly diminished here.