underwriting is the process where the bank ensures that you are a good candidate for a loan. In the case of a mortgage, this process is very important for both you and the bank, because the amount is so large.
What is a Non-Conforming Mortgage in Real Estate?

Non-conforming mortgages do not meet FHFA mortgage amount limits and/of underwriting guidelines set up by Fannie Mae and Freddie Mac.
They usually require more paperwork and a higher interest rate because they are riskier.