You know a magic spell.
The Magic Spell skill doesn't work like a standard skill. A character with 1 rank in this skill knows one magic spell. Each additional magic spell costs 1 rank. When characters adds a rank to Magic Spell, they choose a new magic spell that they now know. Characters never make Magic Spell checks: a character either knows a specific magic spell or doesn't.
Special: the maximum number of ranks in this skill is double the usual maximum of character level + 3 (for example, a 3rd level character has a normal maximum of 6 ranks in a skill, but a maximum of 12 ranks in Magic Spell).
Feat: the Spellcasting feat allows a character to use power points to cast any magic spell they know.