To determine whether you are eligible for a mortgage, a mortgage lender will want to assess a number of different factors before giving their approval to lend to you. Taking out a mortgage generally involves borrowing a significant amount of money over a fairly long period of time. Lenders will want to look at the bigger picture of your personal situation and finances. This is to satisfy themselves that you will be able to afford this commitment.
To help with this assessment, lenders will have their own set of lending criteria. They will apply this to mortgage applications as a guide to determining their appetite to lend. As a borrower, you will generally need to meet these criteria to be eligible for a mortgage from them.