Shooting high is when the spear impacts at a point higher than you were intending ie if you aim at the bullseye and hit the top of the target you are shooting high. Shooting low is the exact opposite. There are a lot of factors that can influence a shot high or low. They can very a great deal depending on the gun and shooter. Aiming is a very objective thing and is very different for each shooter. Two people can fire the same gun and one could miss every shot and the other could look like a world champion.
Muzzle flip usually causes the shot to go low because the tip of the gun moves up from the recoil and pushes the back end of the shaft up.
