An autobid means that you can indicate a maximum bid, after which the system will automatically bid for you until your set maximum bid is outbid. For example, if you want to bid a maximum of 20 euros on a lot, as shown in the image below, you can set this. The system will then keep track of the progress of the bids. When you are outbid, a higher bid will be placed for you immediately and automatically. This takes into account the minimum bid steps that have been set. Also there will never be more than 20 euro's bid.
Note: are you already the highest bidder on a lot and then set a autobid, then you outbid yourself. At that moment there will be a warning stating whether you are sure you want to place this bid. It is not possible to cancel or reverse a bid.