What does it mean when a boy says "keep it our little secret"?

A while ago me and this boys were quite close we would text and chat for hours, one day we both kind of drifted apart. We didn't talk properly for about 3 months we would talk a bit say hi in school but nothing really. A few days again we started texting like we used it started like a normal conversation and ended up it sexting. I know I really turned him on, after we calmed down he told me that I'm amazing with words and said to keep it our little secret, I know for a fact he chats with one of the other girls that I know and he tried to talk dirty with her. I'm really starting to like him, so in the context of this what does "let's keep this our little secret " mean?
What does it mean when a boy says "keep it our little secret"?
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