Every spell has a level from 0 to 9. A spell's level is a general indicator of how powerful it is, with the lowly (but still impressive) magic missile at 1st level and the earth-shaking wish at 9th. Cantrips-simple but powerful spells that characters can cast almost by rote-are level 0. The higher a spell's level, the higher level a spellcaster must be to use that spell.
Spell level and character level don't correspond directly. Typically, a character has to be at least 17th level, not 9th level, to cast a 9th-level spell.