Multiclassing allows you to learn spells and skills of other classes if you so desire. Only players that have not reincarnated can multiclass.
Until you reach level 11, your automatically increase in your primary class, but from level 11 you can save your credits for either the purchase of credit items or to multiclass.
The number of credits required varies depending on what class you wish to gain a level in. You can only multiclass up to 50% of your primary level, but you can multiclass to as many other classes as you wish.