Documents can be locked so that only the person who has locked it can upload a new version of the document. This is helpful if you've downloaded a document to work on it, and want to stop someone else changing it before you upload your changed version. (You don't have to lock a document to edit it if you think it's unlikely anyone else will be making changes at the same time). Alternatively, you could lock a document to stop others editing it if you want to compile feedback and then make the changes yourself, rather than letting them update the document directly.
With Edit in Office, the document is automatically locked when you open it to edit and unlocked when you save it.