The actual description says it increases spell power by 5%. I assume that means all your spells are increased by 5%. So a spell which would have done 100 fire damage now does 105, a heal which would have healed for 100 now heals for 105 etc.
Of course, it may mean it increases masteries by 5%, which is indeed a rather different thing. Sadly there are too many instances in the game where lax use of power and mastery can and do cause confusion.