Whilst I've previously studied a module with the OU, I've never had this. According to the "module result notes" that I received with my module result, the "pending" means "the Examination and Assessment Board is unable to determine a module result in the time available and gives a pending result". They also say that they try to issue results ASAP. From this, I can guess that your result is a difficult case, although that is strange, since the Open University normally publishes mark boundaries for modules. Obviously, the mark boundaries have been set, but they're not sure what mark to give you.
By the way (out of interest), what module is this result from?
R