If you specialize in an arcane school, rather than studying each school equally (as universalists do), you gain an extra spell slot for each level of spell you can cast. You can prepare only spells of your chosen arcane school in these extra slots. In addition, you can prepare an extra cantrip of your chosen school. You also add another arcane spell of your chosen school to your spellbook.
You learn a school spell, a special type of spell taught to students of your arcane school. School spells are a type of focus spell. It costs 1 Focus Point to cast a focus spell, and you start with a focus pool of 1 Focus Point. You refill your focus pool during your daily preparations, and you can regain 1 Focus Point by spending 10 minutes using the Refocus activity to study your spellbook or perform arcane research. Focus spells are automatically heightened to half your level rounded up.
Focus spells don’t require spell slots, nor can you cast them using spell slots. Certain feats can give you more focus spells and increase the size of your focus pool, though your focus pool can never hold more than 3 Focus Points. The full rules for focus spells appear on page 300.