Any new bid you place must be greater than the current high bid. How much greater depends on the current price of the item being auctioned. For example, if you're bidding on an item with a current price of $25, then you must bid at least $26. (Though you can certainly bid more.) The table below breaks out the increments.

Current Price Bid Increment $0.01 - $0.99 $0.05 $1.00 - $4.99 $0.25 $5.00 - $24.99 $0.50 $25.00 - $99.99 $1.00 $100.00 - $249.99 $2.50 $250.00 - $499.99 $5.00 $500.00 - $999.99 $10.00 $1,000.00 - $2,499.99 $25.00 $2,500.00 - $4,999.99 $50.00 $5,000.00 and up $100.00