If you win your first bet, then your entire return (stake plus profit) will be staked on bet two. If you win bet two, then your entire stake plus profits will again be staked when you placed bet three. This is why it's known as a rolling bet.
In-play betting, also known as live betting or run betting, is the process of placing a bet during an event. This could be, for example, during a football match, horse race, golf tournament, or any event that carries a betting market.