Definition of flat call
A flat call is simply calling a bet when a raise is possible. In this situation, you are sandwiched between two players. For example, you are at flop in the button position. You are three players on that flop with the UTG player and the SB. On the flop, the small blind check, the player in the UTG position bets and you call. Here your game will be defined as a flat call since the action is not finished. There is still the small blind in the hand that could raise (make a check-raise).
The symbolism of the flat call
What is a flat call? Of course, it depends on your level, but when a good flat call player is a flat call, he is very strong. It's hard to make a flat call without being very strong. You'll see some fishs's do flat call's when they're weak, but good players will do flat call's when they're strong. For example, imagine the next hand: UTG raises preflop, UTG+1 3bet and you make a flat call on the button. Here, UTG could very well go for a 4bet. The players have a range that is not capped. For this reason, your flat call will show a lot of strength.