MouseDelayFix fixes the mouse delay bug introduced in Minecraft 1.8 (MC-67665), which causes your actual block and entity interaction position to be one tick behind your crosshair.
Simply place the jar file in your forge mods folder as you would with any other forge mod. The jar should work with all Minecraft versions from 1.8 onwards. Delete the jar file to uninstall the mod. The bug was fixed in Minecraft 1.11, this mod will have no effect in 1.11 and up.