The lock statement compiles down to the static Monitor class.  In other words:

lock([SOME OBJECT])
{
  // execute code
}

.. is equivalent to

Monitor.Enter([SOME OBJECT])
try
{
  // execute code
}
finally
{
  Monitor.Exit([SOME OBJECT]);
}

The monitor class provides more detailed control, but it's useful to know that for simple cases like the one above, they amount to the same thing.