Maybe this will help. Go to the bathroom, plug the sink, and turn on the water.
wait...
wait...
wait...
Eventually the sink will fill up, but won't overflow because there is a drain up near the top that prevents it from overflowing. That is exactly what an overflow does.
Now, imagine that the water flowing out of that drain is going to a sump, rather than going to the sewer, and is being pumped back to the sink. The sink won't overflow because the drain keeps it from doing so. The sump won't overflow, because only the water above the level of the overflow/drain goes down there.
Any better?