What is a lump sum contract in a building construction agreement?

What is a lump sum contract in a building construction agreement?
In this form of agreement, the contractor commits himself or herself to execute specified work for a stated "lump sum" or fixed amount. The lump sum might in some circumstances be adjustable on account of a rise or fall in the cost of labour and/or materials, or on account of any variation to the works at the request of the principal, or might include provisional allowances. But generally the contractor is not entitled to claim further money over the stated amount.