'Instant bid' is one-click bidding. No additional confirmation is required. It is similar to regular bidding. 'Instant bid' meets the conditions you have accepted for the specific auction you are participating in. It is faster and easier. So it gives you the best chance to stay on top of the bidding and become the winner!
You set a direct bid yourself by going to your favorites (click on the heart) and using the slider to turn it on. An instant bid is always binding and final. It is not possible to cancel it.