Lippage is the step felt or seen where one tile edge sits higher than its neighbour. It comes from an uneven substrate, from the slight curvature long tiles have after firing, or from uneven adhesive.
Long planks are usually laid with an offset of no more than about a third of their length, because a half offset puts the highest point of one tile next to the lowest point of the next. Levelling clips help keep edges aligned while the adhesive sets.