If I had to guess, using Paladin as an example:

You're a level 50 Gladiator and are about to do some big NM fight. Let's say Ifrit. You equip your job stone and become a Paladin.

You lose all abilities outside the Sword (Gladiator) skill set. (You can't have anything from CNJ, PUG, MRD, etc.) You keep things like Sentinel, Rampart, Obsess, Red Lotus, etc.

You now GAIN Paladin specific abilities like Provoke III, Fealty, Iron Will, Shield Bash III, Cover II, Palisade, Holy, Light Blade, etc. etc.

You lose your ability to meld skills from other jobs, but you gain skills specific to your role, in this case, tanking.