What have you tried? Have you tried looking at the documentation, and following the link I posted? If you tried, then try reading this particular fragment again:

last_error returns a hash with fields that vary, depending on what the previous operation was and if it succeeded or failed. If the last operation (before the last_error call) failed, either:
err will be set or
errmsg will be set and ok will be 0.

If err is null and ok is 1, the previous operation succeeded.