When you download a document for editing the chapter becomes "checked out" by you - and appears locked to other users. (Think of a book being borrowed from the library - it can't be borrowed by anyone else till you return it.)
When you choose "Save to server" from InCopy/InDesign, the document is uploaded to the system and the document is "checked in" and becomes unlocked so other users can access it.
Locking a chapter:
The above image shows what it looks like when another user has a document downloaded for editing. You can choose to be notified when the document is checked in, you can also download a "read-only" version - but you will not be able to download the document for editing until it has been checked in.
The image above shows what it looks like when you have a document checked out. The red padlock icon indicates that the document has been downloaded for editing and is locked. The padlock will remain until you save the document back to the server. If you try to download the document again you will receive a message indicating that you already have the document checked out for editing.
Unlocking a chapter:
The document is unlocked when you save your work back to the server from InCopy/InDesign. If you decide not to save your work - or for any other reason need to unlock the chapter without saving it - you can click on the red padlock icon. After doing so you will see the following dialog:
Note that unlocking the chapter will no longer allow you to upload any work you did on the document.
To proceed with unlocking the chapter click "OK".
If you are unable to unlock a chapter yourself contact an administrator who will be able to unlock it for you.