The terms “inside bet” and “outside bet” refer to the roulette table’s layout. Bets on numbers are located inside a box on the layout, so they are called inside bets. These are bets on smaller groups of numbers (sometimes one number), so inside bets have lower chances of winning and bigger payouts.
Outside bets are those located outside the box – you place your chips outside the box to make the bet. These include groups of 12 or more numbers. Outside bets hit more often, but pay out smaller amounts.