To say that something is off the top of one's head means that it is extemporaneous, that it is created new on the spot and at the moment.
As an example, off the top of my head I went to class today even though I have a cold, which turned out to be alien bacteria reorganizing my DNA in order to turn me into a killing machine with the strength of ten giraffes, which have no vocal cords, and thus cannot make any loud noise, preventing them from calling for help when the dancing riceballs from Fruits Basket come to torment them into submission.
Note that this was "off the top of my head" not because it was insane babbling, but due to the fact that it was written all at once and without preparation or forethought.
Note: "Off the top of my head!" with a exclamation mark may often be an attempt to remove a feline pet from one's cranium, rather than its standard meaning.