These are game notifications sent out from games that you’ve played a certain number of times. For example, the game you are talking about, “Snakes in a Cartesian Plane” recently acquired badges and they sent out a message about it. In order to receive this message, I believe you had to have played the game three times or more.
If you no longer wish to receive these types of messages from that game, you can select “delete” and then “block game”. This will prevent the game from sending you messages again, to help personalize what type of messages you receive. You do not need to reply to them. :)
Hope this helps!