Ok, "For" has several meanings
-used to show purpose or function
a machine for slicing bread
Let's go for a walk.
Are you learning English for pleasure or for your work?
What did you do that for (= Why did you do that)?
-used to show a length of time
I'm going away for a few days.
That's all the news there is for now.
Now see this sentence "could you come down here for a second?"
I guess "For" here means "reason"
I would say "could you come down here for a second?" could mean "could you come down here for a short conversation with me?"
Or does it means "your coming action lasts a short time, not long""?