The batter is awarded first base if the catcher (or any other fielder) interferes with him at any point during a pitch.
If first base was occupied at the time of the pitch, the runner who held the base is permitted to move up one base. This also applies if first and second base were occupied or the bases were loaded at the time of the pitch.
If first base had been empty at the time of the pitch, no additional runners are permitted to advance.
When catcher interference occurs, the umpire will allow the play to progress because the outcome of the play may be more desirable than the interference. In that case, the offensive manager can elect to accept the outcome of the play over the interference.
Watch: Josh Reddick reaches on catcher interference twice in one game.