Each class has a set number of slots they can put abilities from other jobs into. The amount of slots is dependent on the level of the job you're currently playing. The abilities you can put into them depend on the job you're currently playing and the level of the job the ability comes from. For example, a character with the paladin job can use a slot to be able to cast the cure spell from the Conjurer job if the character has raised their Conjurer to a high enough level to have it. (I think conjurer's get it at level 2 but it's been forever and I don't remember, so if your Conjurer is only level 1 your Paladin can't use cure.) A Dragoon can't equip the Cure spell, even if the character has raised a conjurer high enough, because each job can only pick specific abilities from other specific jobs.