When someone says" I am in no-strings-attached (nsa) relationship" what does he/she referring to?

This is not me this is one of my friends and when I asked him that how is his relationship going? He told me that he is in no-strings-attached relationship. So does anyone know what does no strings attached relationship mean?

When someone says" I am in no-strings-attached (nsa) relationship" what does he/she referring to?
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