Average velocity describes how fast is the object moving over a particular interval of time.
However, instantaneous velocity describes how fast the object moves at different instants of time in that interval.
When the average velocity is equal to the instantaneous velocity over all the instants in a time interval, then the motion is said to be uniform motion.
Example: When a car moves on a straight road with same speed, then the average and the instantanoeus velocities are same throughout. This is uniform motion.