This is a technical topic that involves some computer jargon if we drill into it. However, to make sense you only need to know the following facts:

1. If more than one translator edit a segment at the same time, one of them will lose his/her work.
2. To address that issue, the system locks the segment when you open it for editing.
3. You cannot edit a segment that is locked by someone else, and vice versa.
4. The lock will expire in 30 minutes and system will break it then.
5. You can manually unlock the document if the lock expires but is not cleared.

That means you should never leave a segment as being edited when you go for lunch (or even worse, when you go to bed). If you do so, you are preventing other translators from working on it in time. Secondly, the system will unlock the segment and your translation might be lost.

When you open a segment for editing, you will see the below message:

This document is now locked against simultaneous editing. It will unlock when you navigate elsewhere.

That means everything is OK and you can proceed safely. Sometimes you might see a message like below instead.

This document is locked for editing by Tom since Fri, 2009-08-28 05:20.

Typically that means the segment is being edited by Tom and you should not work on it at the moment.

System will periodically clear all expired locks. In case an expired lock persists for any reason, you will see an unblock link following the above message, with which you can manually unlock the document.