A fixed price, sometimes called a lump sum, is a total agreed before the job starts for a described piece of work. If the job takes longer than you expected, you wear the extra time. If it goes quicker, you keep the difference. The price changes only when the work changes, through a variation the customer agrees to, or through an allowance such as a provisional sum that the quote said would be adjusted. The alternative is charging for time and materials as they are used, often called do and charge, where the customer carries the risk of the job running long.
A fixed price is only as fixed as the description behind it. It holds for the work and the conditions written on the quote. That is why fixed-price quotes need a clear scope and a list of exclusions, and why they are hard to give for work you cannot see.
In practice
Customers prefer fixed prices because they can plan around them, and quotes with one total are easier to say yes to than an hourly rate with a guess attached. For the trade, fixed pricing rewards speed and experience: the faster you get at a standard job, the more each one earns.
A painter prices a three-bedroom interior at $3,960, built from an estimate of 40 hours at $85, which is $3,400, plus $560 of paint. On one house the job takes 46 hours, and the painter absorbs six hours, $510 of labour, with no change to the bill. On the next, a tidier house with fewer patches, it takes 36 hours and the painter keeps $340 of time. Over a year, what matters is whether the estimates are right on average. Tracking actual hours against quoted hours on every job is how you find out, and the guide on hourly rate or fixed price sets out a method.
In Quote Desk
A saved quote cannot be edited, so the total the customer accepts is the fixed price on record, dated. If the scope changes later, the change goes out as a separate quote with its own accept, and the original stays as it was. The accept link page explains what is recorded.