I thnk their job is to see existing legislation, and events, and see if they agree with the meaning of the constitution. So, yes, they need to have a clear understanding of what the target legislation/event is, and what the constituion means. They then rule the legislation/event constituional or not.
So, if State A's legislature proposes a law, and the SUpreme COurt rules it unconstituional, then the law is struck down.
Similalry, if the Congress passes a law, and it is found ot be unconstitutional, it is struck down.
Unconstituional would mean inconsistent with the laws in the constitution. A strict contructionist would interpret the consitution narrowly, while one who "legislates from the bench" would read all sorts of things in the constituion that are not there.
I have no idea about the 3/5 of a person, i don't see that in the constitution.
-akhilesh