What does blue portal and orange portal signify?

There is no functional difference between the two colours. They are just two ends of the same portal and like you said, if you enter one portal, you will come out of the other. As spencer added in a comment, the colours help you know which portal you have fired. That way you can avoid firing the same portal twice and moving it when you might not want to.

You can't have two blue portals or two orange portals up at the same time. If a test chamber has a pre-opened portal for you somewhere, you can open a portal of the other colour to take advantage of it.