what is the meaning of HER2+

[Her2Sue]

I am confused. My tumor was Her2+, Surgery came back all clear margins and clear nodes. I am doing Herceptin for the rest of my year time. Does this mean I will definitley get cancer again? Do we even know? I was also er+ 90% I am just curious, because everything I read about her2 says it means poor prognosis! ACKKK! If anyone can let me know LOL I would appreciate it.
Thank you
Susan