This one has got me a few times.
What it means is that the files you are trying to commit are in use by another application OR the repository files at the server end are in use by another application.
9 times out of 10 that application is your anti virus software.
Putting a folder exclusion on your anti virus so it doesn't look at your visual studio projects folder or your subversion repositories folder should solve the issue.
It’s the first thing you should check anyway!