when you engage the button for the second speed. what you are doing is literally changing the gear ratio of the reel, to a lower ratio. what this does is make cranking on heavier drags seem almost effortless. the downside is that if you need to pick up line really fast, low gear will be slow. but, thats easily changed with a two-speed reel, you can just flip the release and be back into high-speed ratio in a split second.
take your rod reel, tie the line to something heavy, crank with in with high-gear one time, and then engage the lower gear , and crank in the same heavy object, you will instantly see the difference.
That is not a stupid question. It is a very smart question- A lot of people choose to live their lives clueless and they are the stupid ones.
I second what jesse and 92 said.