Try and picture it like this..
Your standing in front of a tunel, That tunel has a sealed door that can only open away from you.
Behind that door, is a lot of air pressure thats built up inside, and if you open that door, the pressure of that air will be released.
Now imagine your holding a steel bar, and your going to use this to push open the door.
How much air will escape from that tunel, depends how much strength you push and how prolong you keep the door open.
Now on a marker, the normal dwell setting might be lets say 10.
Now lets say you push the door with the steel bar hard enough to open it for the exact amount of air you wish to escape, that being the same amount of dwell in comparison to the dwell of 10 on your marker.
If the dwell on the marker was at, lets say 2, in comparison with the above example, you will not hit the door. So no air escapes.
If the dwell on the marker was at, lets say, 15, in comparison with the above example, you are not only pushing open the door of the tunel, you are holding it for a small period of time, so more air escapes (in a markers respect, wasting air, less efficient).
It takes time/effort to get the right dwell settings for each marker for efficiency and consistancy.
A higher dwell in most cases will be easier on paint in the winter months (but that depends what marker your shooting ).
With a lower dwell setting, most markers can substain a higher rate of fire (but still need a dwell to release enough air) due to you opening the valve for a shorter period of time, then you would need a higher pressure behind the valve in order to release the same amount of air needed.
There are other settings to consider, but this is the basics of Dwell on a marker.
Hope thats easy enough for you to understand.