What does 'fill power' actually tell me?

Fill power measures loft — higher numbers (700+) mean more warmth per ounce and a fluffier feel. Community threads we read flag that many budget 'goose feather down' inserts are mostly feather and fiber with relatively modest fill power, so they sit flatter than premium 700+ fill down. If you want a true high-loft down feel, check that the listing specifies fill power, not just fill weight.