It means You are saying that to me as if you mean it only about me, but I bet you say that to a lot of people. It is commonly used in a flirting situation where the first person makes a compliment, and the second person wants to, in a light-hearted way, accept the compliment but also appear modest. Example: She: Wow, you really know how to make me feel special! He: I bet you say that to all the guys. The phrase can also be dismissive. In other words, the meaning can be "You said that but you didn't really mean it." The difference in meaning can be discerned by the tone and the situation.