When a quarterback looks at the defensive players to see if they will be blitzing and from which side of the offensive line. The quarterback's pre-snap read looks at how they're lined up and what stance they're in.
A quarterback's ability to read coverages is critical because it helps speed up his decision-making process after the ball has been snapped.
Pre-snap read can also refer to defensive players looking at where the offensive players are lined up so as to better prepare for a run or passing play.
Some things a quarterback will look at pre-snap are:
The difficulty is that defensive players will usually change their alignment to keep the quarterback guessing, such as stacking guys up on the line of scrimmage to show blitz, but then backing off into pass coverage after the ball has been snapped.