The cap is the max ability for a sailor like, your EBVE gunner can only be so accurate. When you hit this cap it basicly means, you won't get any more ability.

Let's say we have 2 sailors.

One with a +13 growth and the other +11.

Assuming both have the same ammount of vets experts and recruits.

At one point the +13 sailor will reach the cap and will stop improving with the +11 sailor usually catching up and becoming just as good later on.

Note that not all abilities have a cap and not all capable abilities are easy to cap

Torpedo reload speed usually caps fast.
Gun Accuracy/Reload usually reaches a soft cap at around level 80 (and will only inprove in small incriments upon leveling up).
Engine overheat ability has a cap of sorts (you can go only at a certain speed based on cruise speed) however overheat time has no known cap.
A repair speed cap also exists however it's ludicrously hard to reach unless you play as RN or/and raise a specialised crew.