The default bid is your maximum cost-per-click. It's the most you'll pay per click for ads in an ad group.
How it works: We'll use your default bid for clicks on your ads unless you set unique targeting-specific bids. The closer you get to your budget limit, the less your ads will show.
What to do:You should set a bid that allows you to get the most value out of a click while staying within your budget.
Example: On average, one in five clicks on your ad leads to a sale worth $5. Because you don't profit for click costs of $1 or more, you set a default bid of $1.