What does pull up your socks mean?

It is a phrase that is like saying, "Get over it" or "Quit crying over spilt milk."

Pull up your socks and get on with your life or job or whatever.
Pull your socks up means - you should do better

You had better pull your socks up next year or you will not pass your exams.
To improve Behaviour
Means that you should make more of an effort!