what is the term banking in a circular motion?

It is done to the road so that vehicle can move on a curved track, without skidding. This is done so that a component of weight of the vehicle provides the centripetal force and the vehicle does not depend entirely on friction for the centripetal force.

For a curved track with radius 'r' with maximum speed 'v', the track is banked through an angle θ given by

                                              tan θ = v²/rg

                                            θ = angle of banking