"hg push" errors with "could not lock repository"

[Ethan Mallove]

Hello,

What "lock" is "hg push" referring to below?

  $ hg push
  pushing to /foo/bar
  abort: could not lock repository /foo/bar: 
  Permission denied

Is there an "hg cleanup" command (like "svn cleanup") that
removes locks?

All files in the "hg" area are group-writable, and
directories are everyone-executable. If I 777 everything,
things seem to work, but this is obviously the wrong way to
go.

Thanks,
Ethan