They are collectible cards, a lot of games on steam have them. You play for a while to naturally gain some of them, you have buy or trade for the rest. I'm sure most of us know about the existence of the cards.
I think what happened is that the dude has his computer set in Korean, and when he tried to reply with a quote of what you said, his search engine translated your comment into Korean as well, but bugged out and just put your quote twice, alongside his reply "Okay thanks "