When Subversion modifies your working copy (or any information within .svn), it tries to do so as safely as possible. Before changing the working copy, Subversion writes its intentions to a log file. Next it executes the commands in the log file to apply the requested change, holding a lock on the relevant part of the working copy while it works—to prevent other Subversion clients from accessing the working copy in mid-change. Finally, Subversion removes the log file. Architecturally, this is similar to a journaled filesystem. If a Subversion operation is interrupted (if the process is killed, or if the machine crashes, for example), the log files remain on disk. By re-executing the log files, Subversion can complete the previously started operation, and your working copy can get itself back into a consistent state.

And this is exactly what svn cleanup does: it searches your working copy and runs any leftover logs, removing working copy locks in the process. If Subversion ever tells you that some part of your working copy is “locked”, then this is the command that you should run. Also, svn status will display an L next to locked items:

$ svn status
  L    somedir
M      somedir/foo.c

$ svn cleanup
$ svn status
M      somedir/foo.c

Don't confuse these working copy locks with the ordinary locks that Subversion users create when using the “lock-modify-unlock” model of concurrent version control; see The three meanings of “lock” for clarification.