Perashat Ki Tesse has more Missvot than any other Perashah in the Torah, with 27 positive commandments and 47 negative commandments, for a total of 74 Missvot. Among these Missvot are the prohibitions regarding charging or paying interest on loans between Jewish people. The verse in our Perashah says, לא תשיך לאחיך Halakhically speaking, this is the prohibition of paying interest. According to this explanation, תשיך means to be bitten, so to speak, meaning to pay interest as opposed to לא תשך which would mean not to bite, meaning not to charge interest. Therefore Rashi, based on the Gemara, explained that the prohibition of charging interest is found in Perashat Behar in the verse that said את כספך לא תתן לו בנשך
The Ramban pointed out that prohibiting the one who is losing money in a transaction is particular to the laws concerning interest. Normally if a person agrees to lose money in a transaction it is permissible. But here the Torah also restricted the borrower from paying interest since charging interest is so common it could never be prevented unless the borrower was also restricted from paying it. However Ibn Ezra's explanation of the straightforward meaning of לא תשיך is the prohibition of causing the borrower to pay interest since he is not permitted to do so. Ibn Ezra also wrote that some explain that there is no difference in meaning between תשך and תשיך, and that they both mean to bite, or charge interest. We find this explanation of the straightforward meaning of the verse in the commentary of the Rashbam. So the Halakhic explanation of לא תשיך is not to pay interest (be bitten), however some explain the straightforward meaning of the verse to not cause the borrower to pay interest, meaning not to charge interest.
We also find various explanations of the next verse that says לנכרי תשיך ולאחיך לא תשיך According to HaRambam, based on how he understood the Sifre, a Jew is required to charge a non-jew interest on a loan unless he expects to get some other benefit from the borrower for making the loan. The Sefer HaHinukh explained that this is to diffrentiate between our fellow Jews and non-Jews by only extending the kindness of an interest free loan to a fellow Jew. However Rashi, based on the Gemara in Baba Messi'a explained that the Torah is not requiring Jews to charge non-Jews interest. According to this explanation, this verse, which is in the form of a positive commandment, is connected to the prohibition of not charging a Jew interest. Meaning when one charges a Jew interest they violate both the negative commandment of not charging a Jew interest (ולאחיך לא תשיך) and the positive commandment of לנכרי תשיך which means you may only charge a non-Jew interest, not a Jew.
According to the Ramban this verse only permits charging the non-Jew interest, it does not require it. The point of this verse is that even though one may not steal from a non-Jew, one may charge a non-Jew interest because that is something both parties agree to. Not charging a Jew interest is a higher standard where the Torah obligates us to perform kindness to our fellow Jews.
Lastly based on תשיך meaning paying interest as opposed to charging it, the Seforno explained that the Torah is allowing a Jew to pay interest to a non-Jew. The Seforno added that if you take a loan from a non-Jew and agree to pay interest, but then refuse to pay the interest, it is a desecration of HaShem's Name. Whereas if the lender is a Jew it is not permitted to pay interest even if you agreed to pay interest when the loan was made.
So לנכרי תשיך either means you must charge a non-Jew interest, or you may charge a non-Jew interest, or you may pay a non-Jew interest. Or it could just add a violation of a positive commandment to the prohibition of charging a Jew interest (since you may only charge a non-Jew interest). Practically speaking we have accepted the rulings of HaRambam, requiring one to charge a non-Jew interest unless the lender is getting another benefit in return for making the loan.