This is because someone "Checked out" a document. It's something called "Versioning".
You will find VERSIONS on the left hand side, usually at the bottom:
If the document is versioned, this is what you will see:
To continue editing the document, or to change it back because this was a mistake, select the version that says "1.1" with the lock on it.
You will be able to cancel it or check in the document.
If you check it in you will receive this prompt:
*VERSIONS CAN BE DELETED IF THERE WAS AN ACCIDENTAL "CHECK IN" OR IF YOU NO LONGER NEED A VERSION:
CLICK THE X NEXT TO THE VERSION