It's not a stupid question. But 'jar' files are java programs, and there is no java support on the 770 (nor the N800). For now, at least.

Edit: Or actually, there used to be a 3rd party effort for java on the 770, I even had it installed on my N800 for a while but never tried any java program so I'm not sure what it could do (or not do). In any case you might try searching for 'java' on the forum. Mind, there will be lots of irrelevant hits too.