Come on is a fast phrase which is meant to persuade someone into understanding a different perspective.
It can be used to attempt to help someone agree/convince them that you have their best intentions in mind.
Come on can also
mean that somebody is being too
slow, and they need to do something faster.
"Come on" isn't meant to be used
literally.
However, the phrase is common, yet vague, nondescript, and should not be used.