It prevents the arrow from disappearing and replaces it with a special crystal projectile.
Normally, when arrows are used in Minecraft, the player shoots them out of the bow and they disappear. But Infinity is a mod that adds a new type of item to Minecraft called “Crystals”. Crystals act essentially like arrows but have some different variables as well. If you fire one of these crystals from an Infinity enhanced bow then your arrow won’t disappear and it will make players show up on radar. Other than that, nothing happens! Pretty useful for PvP battles or hunting down those pesky guardians or creepers without having to worry about where they went off too after shooting them down.