A mortgage offer is a formal document that confirms a lender is happy to loan a buyer the money required to purchase a property. It will detail in the document the exact figure the lender is willing to advance to the buyer.
Before the mortgage offer is given, the lender will first of all carry out certain checks such as valuing the property, verifying the buyers income and performing credit checks. This is to make sure the buyer can repay the loan given, that they believe the asking price for the property is reasonable and the property is not a risk to them.
The most important thing is to read the mortgage offer carefully as it will contain all the various conditions of the mortgage. If you don't agree with any of the statements on the offer, you must speak to the lender, as once this is accepted, it becomes a legally binding contract.