All spellcasting classes have a number of spells of a given level they can
prepare each day. Theses are referred to as slots. As you advance in levels,
the number of slots you have increases, and the highest level spell you can
prepare increases as well.
Example: A level 1 priest has one 1st level spell slot, in which he can
prepare a single spell and subsequently cast it. He has to rest to regain
the ability to cast that spell. By the time he reaches level 6, he will have
three 1st level spell slots, and will gain one 2nd level slot. Then he will
be able to prepare and cast three 1st level spells, and one 2nd level spell,
before having to rest or meditate to regain his spells.
The number of slots available to a character can increase if their ability
score in the primary stat for their class increases. High intelligence for
wizards, high wisdom for divine spellcasters, and high charisma for sorcerers
and bards will add bonus slots to a casters normal slots at each level.
In addition, in order to cast a certain level of spell, the caster's primary
stat must equal 10 + the level of the spell. So, a 6th level wizard spell
would require and INT of 16 to cast.
Conversely, should a character's primary stat drop below 10, he or she will
lose the ability to cast spells entirely until the ability once again rises
above 10.