what is opposition school?

[jonathan]

The game explains specialist schools but not opposition. what is it?

[Kommissar K]

A wizard that chooses to specialize in one school of magic must select two other schools as his opposition schools, representing knowledge sacrificed in one area of arcane lore to gain mastery in another. A wizard who prepares spells from his opposition schools must use two spell slots of that level to prepare the spell. For example, a wizard with evocation as an opposition school must expend two of his available 3rd-level spell slots to prepare a fireball. In addition, a specialist takes a –4 penalty on any skill checks made when crafting a magic item that has a spell from one of his opposition schools as a prerequisite. A universalist wizard can prepare spells from any school without restriction.

You pick two schools, and have trouble preparing spells from them. Its the cost you pay for the benefit of being particularly good with one school.