Pronation refers to the travel of foot as the heel lands and rolls towards the ball of the foot.
As runners and walkers, we can over-pronate, under-pronate, or be balanced.
Over-pronation, where your foot pushes off almost completely from the big toe and second toe. This causes the shock from landing to not spread evenly.
On the other hand, under-pronation is where your heel lands and rolls to the outside of the foot. This causes your foot to bear most of the weight on the outside.
Most over- or under-pronate. Most quality shoe stores will help you determine whether you over- or under-pronate. Or you can do it at home on a the sidewalk.
To do it at home, simply get your feet wet and take a natural stride on the cement while walking. If the watermark shows very little inside foot, then you under-pronate. If you see the watermark on the inside of the foot, then you under-pronate.
Now, do the same thing with a slow jog run. Lastly, have a friend stand behind you while you are running on a treadmill in bare feet. Use the eye-ball test to confirm what the first two tests show.
Again, our recommendation is that you get computer tested.
The pronation difference is hardly noticeable over short distances and shorter times, but becomes very noticeable over longer distances and greater time.
Knowing your pronation will help you buy a better shoe and will keep you running longer in terms of mileage and longer in terms of age.