Auto Bid is a feature that lets you set a maximum bid amount by using the (+) button in order to automatically place bids over other bidders' until the maximum is reached. Your bids are up by the minimal possible unit (1 unit).
For example, if an item has a current value of 1,000,000 yen and you set the maximum bid amount at 1,080,000 yen, you will first place a bid for 1,010,000 yen.
Afterwards, if another auction buyer places a bid for 1,040,000 yen, you will automatically place a bid for 1,050,000 yen.
For more details, please take a look at the "How to Use the ALLSTOCKER Auction Website" section.