DEAR FREIND. :)
This could be explained by giving a example, suppose you, on a motorbike planned to travel 1km on a straight road.
suppose you are asked to travel that 1km in 60 sec or 1 minute.
considering following two cases.
CASE I  : There are no obstacles on road.
             you''ll straight forward start your bike and will cover that kilometer with a constant speed of 60km/h.
             this meant you performed a uniform motion.
CASE II :OBSTACLES ON ROAD.
             suppose there is traffic on road, so you''ll have to apply breaks many times during that journey but you have given
             a deadline of 60 sec, but apllying breaks will decrease you velocity ofcourse, so you''ll have to cover that lost speed
             by speeding up more when you get space. 
There if you managed to get there in 60 sec, this meant wherever you varied speed in between, you managed to cover 1 km in 60 sec. same as in case 1, therefore here your average velocity will be 
 
VAV=   Total distance covered / total time taken.=60 km/h
in case 1 also you covered 1 km in 60 sec, therfore giving average velocity of 60km/h
but unlike case 2 , you were travelling at 60 km/h at all times. 
WHEN A BODY IS PERFORMING UNIFORM MOTION ITS VELOCITY IS SAME AS AVERAGE VELOCITY AT ALL INSTANTS.
 
fORMALLY
 
Commonly , Instant velocity is reffered as Velocity, which means the velocity of any object with respect to non accelerating reference frame, at any intance of time.
i.e Velocity of object at a particular time, say tth second.
Where as AVERAGE VELOCITY is the mean of all velocities attained by a body during a time interval, say ''t'' seconds. i.e total distance covered in ''t'' seconds. Which doesn''t say any thing about what was the velocity during the time interval.

IF a body is performing uniform motion, it''s velocity doesn''t vary at any intant during that time interval, therfore average velocity is eqaual to instant velocity at all times.