The relative power of a spell is indicated by its level. A spellís level also indicates whether a particular spellcaster is capable of preparing and casting the spell.
Spell Slots: Every spellcaster can prepare as many spells every day as he or she has spell slots. (See the advance class description for details.)
Spell slots have levels just as spells do; a spell slot of a certain level is designed to hold a spell of that level. Itís possible, however, to place a lower-level spell in a higher-level spell slot if you desire. A character doesnít have to fill all spell slots with prepared spells every day (although it usually makes sense to do so).