From Chabad.org: The laws mentioned yesterday apply to fields and homes in unwalled cities. Homes in walled cities, on the other hand, may only be redeemed up to one year after the sale; otherwise they become the permanent property of the buyer. Another exception to these rules is the property allotted to the Levites, which are always redeemable. We are commanded to assist our brethren by coming to their aid before they become financially ruined and dependent on the help of others. We are also forbidden from charging interest on a loan to a fellow Jew.
When describing the rule not to charge a fellow Jew interest the Torah uses two words to describe the interest, which the Rabbis infer to make it a double Aveira (sin) to do so. This law comes on the heels of letting the Levi’im (Levites) buy back their fields, helping another person (Jew or non-Jew) as they falter but before they fall, and now this. This progression seems to be the blueprint to build a community that cares and helps each other. And the bottom line is to make sure that you don’t do it for your own benefit. Hence the double-underline when discussing the interest prohibition.