A deposit match adds bonus funds equal to a share of your deposit. A 100% match on $200 gives $200 cash plus $200 bonus.
The bonus is locked until rollover clears. Rollover means total stakes required before withdrawal. A 5x rollover on a $200 bonus asks for $1,000 in settled bets.
Most sportsbooks apply the match to the first deposit only. A few spread it across the first three deposits, with a lower percentage each time.