Flat bet means betting the same amount all the time. Most players don't like to bet this way because they figure if the dealer wins more hands then them they'll never win any money. Also players feel it's boring betting this way. They prefer the thrill of sometimes betting more in the hopes that when they do so, they will win the hand and make a nice profit.
Here are the facts on flat betting. If you bet say $5 on every hand in a standard multiple deck game the house will have an edge of 0.5% against the skilled basic strategy player. That means you will lose on average 0.5% of every wager you make. So if you bet $10 on every hand and average 80 hands per hour you will have made a total of $800 worth of bets. The casino expects to earn 0.5% of the $800 or $4.00. Of course the more likely result is that you'll win or lose much more than $4 after an hour of play. But on average you can expect over time to lose at the rate of $4 per hour.