Auto Top-up charges the card you last used for payment to buy another Credit Package when you drop below a certain level of Credits. (See Term 6.9.4 T&Cs) This means you never have to step away from an auction to top up your Credits.
Lets say the current Credit Package purchased is a 300 Credit Package costing 29.99 GBP. You will automatically be given any running Promo Code for your purchase and, if no Promo Code exists, you will be given 10% additional Credits.
With Autobid and Auto Top-up enabled, your bids will just keep coming!
Auto Top-up initially has a cap to stop each day. However, you can set this cap higher if you are bidding on multiple Auctions.