Its a CS/programming term.
The short story is that programs (esp threaded programs) have to ensure that no other programs (and no other member threads) attempt to access the same resource (be it cache/memory/variables) at the same time. That is mutual exclusion -- When one thread/etc has it then noone else can get it...
blah blah most common form is the mutex readers/writers lock some OS's use. This one doesnt allow data to be written to a variable while users/programs are reading it, and doesnt allow reading while its writing...
Now why is your program doing it? No clue... windows xp?