What does it mean?
A bill of quantities lists described items of work with their units and measured quantities. Tenderers add rates and amounts so that prices can be compared against a common scope. A bill is only as reliable as its drawings, descriptions and measurement basis; a quantity without a clear description can hide a significant difference in scope.
In practice
A concrete item might state 20 m³ of a specified grade in foundations. At 150 per m³, the extended amount is 3,000. Check whether placing, finishing and associated work are included in that rate.
Keep in mind
For a project record, note the document reference, revision and agreed measurement or contractual basis beside the quantity. Keep assumed information visibly separate from confirmed information. If two parties use different definitions or inclusions, resolve that difference before comparing amounts. These explanations are general estimating context; the wording of the applicable contract and its administration process must be checked for the particular job.