It is when there is relative movement between a rotating object and the ground at the point of contact so the condition

translational velocity  $=$ radius of object $\times$ angular speed of object

is not  satisfied.

Push down too hard on the accelerator pedal and the drive wheels of your car will spin with the car and produce a lot of smoke due to the tyres getting very hot because there is relative movement between the tyres and the road - slipping.