It means "to tell you" or "to say to you".
Here are some examples:
* ¿Qué quieres decirte? (What do you want to tell me?)
* Tengo que decirte algo importante. (I have to tell you something important.)
* Quiero decirte que te quiero mucho. (I want to tell you that I love you very much.)
It's important to note that the informal "tú" form is used for friends, family, and people you know well. If you are speaking to someone you don't know well or are in a more formal setting, you would use the formal "usted" form, which would be "decirle".