It was a plot to blow up Parliament and (maybe) the king if he was sitting in it when in session.

Guy Fawkes was amongst a group of Roman Catholics who tried to do this because of the increasingly harsh laws on recusancy (not attending Anglican i.e. not Catholic mass and ceremonies). They basically disliked the break from the Pope (happened way before) and were scared at the fact that a Scot (James I), who was more Protestant and puritanical than his predecessor, was on the throne. Overall, their motives seem quite simple and, in the grand scheme of things, it is not that significant an event, but it was one of the last few Catholic insurgencies in the Tudor-Early Stuart period in England