When a file such as a word or excel document is opened from a network share it is locked for editing so that only the user that has opened it can make changes to it. If another user opens it, it will open in read only mode to prevent edits.
This behavior is correct and is usually just fine but sometimes after the first user that opened the document has closed it, Windows does not properly release the lock and anyone who then tries to open the file including the original user find that it is locked for editing.
To get around this you need to login to the server where the file is located so you will most likely need administrative privileges.
You will then want to open:
You can now scroll down till use see the file in question Right click it and select "Close Open File"
**** Be sure that no one has the file open or the user may lose data if you close an open shared file ****