New to this - what is star level do in clash royale?

star levels cost star points, not gold. you get those mostly from upgrading cards, and once a card is maxed any extra copies turn into star points instead. so spend them on cards you actually main, not ones you're about to drop from the deck.

it's a skin, that's the whole feature. gold outline and a sparkle when it drops - no damage, no hitpoints, nothing that changes a match.