This happens due to the system, it means that there is an edit already awaiting approval. If you later have more reputation (over 2000) you will be able to review the edits happened there and choose action accordingly.

You might want to have a look at this post happened earlier today:

http://meta.askubuntu.com/questions/15531/editing-a-post-when-there-is-already-an-edit-waiting-for-review