How Do a High Pass
You need to hold LB/L1 for a high pass just after you hit the receiver icon.
The High Pass allows your receiver to make a jumping catch, hopefully, to get the ball away from any defenders. These are particularly good if you’ve got a receiver with some height (like the taller tight ends in the NFL) or if they are quite agile.
How Do a Low Pass
Receiver Icon + Hold L2 or LT
The Low Pass just makes things simpler for your receiver to catch the ball lower to the ground and away from defenders.