This file simply contains the PID of the user's running aMule instance.
The following is the contents of a valid muleLock file:
This file is created when starting aMule and it writes its PID number in it. When aMule exits, it deletes the file.
When aMule starts, it checks if the muleLock fiel exists. If it does, it reads it and checks if the number it contains is the PID of a running process. If it is, then it closes itself to prevent two aMule intances running simoultaneously.
Notice that aMule will not check that the process corresponds to a real aMule process. Any processes running with the PID contained in the muleLock file will prevent aMule from starting.