What are tumor markers?

THis is probably a stupid question, but I have not a clue what any of you are talking about when you say that you've had tests for your tumor markers. I don't think that I have ever had this done. I am 24 years old and was diagnosed in April 2001 with stage 2, grade 4, breast cancer. 1 of 13 nodes were positive, my tumor was 2cm, estrogen-, but strongly HER2 positive. Any info you could give me on the subject would be greatly appreciated. Thanks so much and God Bless all of you! Carrie