The number 420 (4:20 am/pm) is especially significant amongst marijuana users and enthusiasts. It most likely stems from the novel ""The Count of Monte Cristo"" in which the character(s) smoke some marijuana and refer to something lasting 4 months and 20 days (I don't remember exactly.)
69 is when two people love each other and stick their genitals in/over each others mouths. At the same time. This is usually achieved when one partner is flipped upside down/rotated 180 degrees in relation to the other partner. That position is comparable to how the number 6 is an upside down number 9 in the majority of typefaces.
in other words the
420 stands for April 20. It's an unofficial weed appreciation holiday. Some people like to mock weedheads by typing 420.
69 is a sexual position. Many immature people like to put it in their usernames, so some people on Reddit mock it by typing 69.