If you shake something off, you get over it and move on. Usually it is something painful, either physically or emotionally. "That guy crashed into me pretty hard! It hurt some." "Shake it off! We need you back in the game."

It is like a dog that shakes off water after getting wet. You just extend that image to anything that sticks to you, like annoyances, grievances, etc.

As I feel, it means "let it go" , "forget it" or "get rid of it". But I'm a learner not a teacher like others.

"Shake it off" basically means "get back up and keep trying". It is usually said when someone hurts oneself while performing a physical activity and is used to indicate that the speaker doesn't think the pain is too great to continue. The phrase is commonly associated with teachers of Physical Education here in America.

To free oneself of; get rid of: We shook off our fears.