Check for running process

I am looking for a way to check for a running process  specifically msiexec and then when it completes to run another program...   I have a start I think that I got from a script to start a process if not running but I don't quite understand what its doing.... heres what I have now....  

:TOP
for /f "usebackq tokens=1" %%a in (`pslist ^| find /i /B "msiexec"`) Do Set ProcessRunning=%%a
If "%processrunning%" == "msiexec" Goto TOP
:End