gitの質問です。

> git push
Counting objects: 11, done.
Compressing objects: 100% (6/6), done.
Writing objects: 100% (6/6), 717 bytes, done.
Total 6 (delta 4), reused 0 (delta 0)
To [email protected]:foobar/project.git
2c1d0b7..2bcaea9 master -> master
! [rejected] README -> README (non-fast forward)
error: failed to push some refs to '[email protected]:foobar/projet.git'

これってどういうエラーですか？何が原因？