Simple: American service has a peculiar quirk in that servers are paid (in 95% of the country) around 2 dollars an hour, and are compensated in tips instead. At minimum, 15% is expected as a tip, even for bad service, and good service has an expectation of 20%. Truly bad service should be spoken to with the manager, rather than simply leaving without any tip.

Note that servers expect you to tell them if something is wrong, so that it can be corrected.

If you have a discount, you are still expected to tip on the full amount.

When you look at the price, add about 9% (depending on locale) for sales tax and another 15% on top of that for the tip. (Tip used to be pre-sales tax, but is considered post-sales tax today.)

Many people tip 20%, but it is not required. Fast food has no tipping. Starbucks and other coffee places, a tip is appreciated but not required. Bars expect a dollar per drink.