Income Tax Returns

I recently applied for a loan through Rural Development and today they sent me a form in the mail that said.. Certification of exemption from filing federal income tax returns.
then it says: i hereby certify that i am exempt from filing federal income tax returns. then there is a spot for me to sigh. i don’t understand what this means.

Do i have to submit Federal income tax returns for the three most recent years or just one if i am sponsoring someone to come up to usa .Plus are there any requirements for permanent resident to sponsor someone to come up to usa.