"Once I'm seventeen, the protective charm that keeps me safe will break, and that exposes you as well to me."
In this paragraph Harry is talking to the Dursleys about a charm his mother put on him. When Harry was 1 years old and Voldemort came to kill him and his parents, his mom basically protected him from Voldemort and when she did so a charm was put on Harry out of eternal love. But when Harry turns seventeen the charm will break, and he will be no longer safe. So he is explaining this to the Dursleys saying Voldemort will come after him the second he turns 17 and if he can't find him he will torture the Dursleys into fessing up.
I found this interesting, how after all the Dursleys did to Harry he would still want to protect them. It's like what Harry's mother did for him. The fact that knowing someone died because you didn't go to the lengths that you could of to save them. If Harry's mother had just stayed away from Voldemort how would she feel for the last seconds of her life knowing her son had died because she didn't have the courage to protect him.
Its funny how one thing affects another. And what if the roles were reversed? Would the Dursleys do this for Harry? What is real love? Harry doesn't 'love' the Dursleys! They have treated him horribly throughout the past years! But he knows they are family, and it might not feel like it but they are connected. Harry had a mother that he may not of known but had a very strong connection to! And Aunt Petunia had a sister (Harry's mother) who she hated for being a wizard and taking away the spotlight from her, but thats like a big fight you still love each other.