A maximum bid in cost per click (CPC), is the highest price you are willing to pay for a click on your ad. The way maximum bidding works is, if someone clicks on your ad, you will never have to worry about the cost of that click going over your "maximum bid". For example, if you paid $3 for an ad bid, your actual CPC will never surpass $3. In CPC you are able to choose between manual bidding, which is where you determine your bid amount. Or you can choose automatic bidding, where you set a targeted budget and AdWords makes sure you do not pass that max.
E.g. “I’ll bid as high as $2.25, but that’s my maximum bid - I’m not made of money!”