Capping is when you bring the polish over the free edge with your brush. Some peeps have thicker nails than others .... a bit like the edges of a piece of paper as opposed to a piece of cardboard. The cardboard is thicker. It is that edge that you need to cap!
When you load your brush with polish and bring it down the length of the nail from the eponychium, don't stop at the free edge. Continue to pull the brush down over the free edge so that it gets a coating of polish as well. If you have run out of polish, or it is drying out at this point, quickly dip your brush back into your bottle, only loading the tip this time. Then go back and run it across the lip of the free edge from left to right. You have to be quick though, as the polish on the nail is already setting.
I hope this makes sense. Chipping can begin because that edge is not sealed. Imagine a piece of pottery, where the glaze is chipped. Moister gets in through the chip, wets the pottery and the chipping gets worse.
HTH x