From Minecraft Parkour Wiki

This procedure is intended to make the player hop over low obstacles. The fatal flaw is that the bounding box is reset to the player's position at the start of the tick, and then applies stepping. If the player was airborne at the start of the tick, the stepping maneuver is initiated above where it's supposed to. In that case, the bounding box might not get lowered enough and stops mid-air. In this scenario, the glitch is called a Blip-Up