I know what you did = I know your hoe ass cheated
So I'm let you taste her = taste another girl's vaginal juice on his dikk
The whole song is about how, The Weeknd found out about his girl cheating on him, so in turn he cheats on the bytch and the love is gone and their acting like everything is peachy. Their both lying to the other, even though they both know what the other has done. He (The-Weeknd) knows what she did and he’s telling her that it didn't break his heart even though she think it did ! but little does she know he did the same thing. They are both hiding their true emotions and feelings behind smiles.