A Real Estate Broker’s formal opinion regarding the value of a particular property. The broker looks at comparable properties (or "comps" for shorthand). A savvy seller will look at two different sort of comps when deciding on an asking price.
First, look at recently sold comps - the more recent, the better. The final sale price of a home isn't known until closing, when the documents are recorded with the county. At that point it is a matter of public record. Sold comps give you your current market value.
Then, look at homes under contract or still on the market. This will tell you your competition. You can then decide how to best balance speed - and certainty! - of sale against getting top dollar.