Multiclassing is the process of taking on multiple classes of the same level, usually in order to gain new abilities and powers. In order to do this, one must first choose a primary class, and then allocate experience points to the secondary classes.
When multiclassing, one must remember that they only have a certain number of spell slots available to them at any given time. This number is based on their level in their primary class.
When multiclassing, it is important to remember that you will always be able to cast spells from your primary class, but you will not be able to cast spells from your secondary classes until you reach levels in those classes that allow you to do so.
For example, a character who is a level 5th-level fighter can cast spells from the fighter and paladin classes, but cannot cast spells from the wizard or sorcerer classes until they reach level 8th-level as a fighter and level 10th-level as a wizard or sorcerer.