This is the document issued by a mortgage lender to a prospective borrower following approval of the mortgage application.
Your mortgage offer will set out in detail the conditions on which the mortgage is being made available to you and the mortgage term - the length of time over which the loan is being made available.
Mortgage offers are typically peppered with legal terminology such as mortgagee (the lender) and mortgagor (the borrower) and the contract will set out whether the lender requires you to buy mortgage protection insurance.
You should study the mortgage offer carefully! These documents are normally valid for a limited period of 3-6 months but they are not irrevocable. This means the lender may still withdraw the funds if there are any material changes in your circumstances (such as redundancy) before the loan is completed.
If the loan does go through, both you as borrower and the mortgage lender are bound by the conditions and terms outlined in the mortgage offer.