A Lava Hop is a trick that gives a one-frame window for Link to jump off of lava, potentially allowing new areas to be reached.
First, you must jump onto the lava while triggering some sort of cutscene (rock explosions, Goddess Cubes, etc). After the cutscene plays, you will have a single frame to input a jump; either front hop, sidehop, or backflip.
The reason this works is due to the hierarchy of triggers; triggers are executed in the following orders:
Thus, assuming a cutscene intervenes, you'll always have one frame afterwards to input a jump before the lava trigger activates.
Due to the difficulty of timing the single-frame input after the cutscene, Lava Hop is generally considered a TAS-only trick. It does have one important application in the Earth Temple, where it can be used to skip the boulder-rolling section.