it's oral sex. the circular part of each numeral is the head and the tail is the body. this sort of oral sex is requited, with both people giving attention to the other person's genitalia.
Guy 1: "I 69'd last night with Jessica and it was awesome!!"
Guy 2: "Nice one."
Think of the circles in the number 6 and 9 to be a human head. Now image the other part of the number 6 and 9 to be genitalia. As you can see, the 6 is shoving it's cock down the 9's throat will the 9 is getting it's pussy licked by the 6. Essentially, a 69 is when two loving human beings position themselves in such a way that looks like the number 69, and allows them to "Go down on each other" simultaneously.