A push in sports betting is when the bet lands on the exact number offered by the sportsbook, and your bet gets refunded. Say you've bet on the Chiefs as 10-point favorites over the Chargers, and K.C. wins by exactly 10 points. Whatever amount you bet is returned to you in full.
Every sportsbook will cancel your bet if the player is inactive per the league or team's official injury report. That's the case if you bet the over or under. Some books will honor the bet if the player is active, even if he or she doesn't play a single second.