Flat bet betting for baccarat and other games means that the amount of your bet does not change. If you are normally a $5 bettor, then every bet you make will be $5. You might switch from bets on the player, to bets on the dealer or even skip a bet. But flat bet baccarat players always make the same bet amount regardless of the situation.
On the flip side, a progression bettor will change the amount of their bets depending upon whether or not they are currently ahead of the game or if they believe that they will win the next hand.
In land based casinos where there is manual deck shuffling, some players also keep track of how many times the deck is shuffled. These player also sometimes believe that a truly random deck has to shuffled twelve times in order to contain a real random condition.