Literally, if you come down, you are physically moving in a downward manner.
Ex: When you're finished, make sure you come down the ladder carefully so you don't fall and break your leg.
Another meaning for come down is travel to the south, or to somewhere less important.
Ex: We just heard from Uncle Johnny, who lives up near the Scottish border, and he said he's going to be coming down to this area next month. I can't wait to see him!
You can say that a person is coming down on you if that person expresses displeasure with you.
Ex: The principal really came down on me after I missed school for the tenth time this month. I begged him to be merciful, but he suspended me for a week.
When you want to talk about the most essential part of a situation, you can say that it comes down to it.
Ex: When it comes down to it, the one thing all photographers need if they want to succeed is a good eye.
Ex: My mother always told me that if you want to succeed, it all boils down to pride. If you like yourself, then others will, too.
Ex: After I had looked at all the information I had to think about it for a while, but eventually I was able to cook it down into a single theory.