This is a position you take when you want to try mutual oral sex simultaneously.
They call it position 69 because the head and legs point in a different direction, looking like a 69.
You can do this side to side or with one person on top of the other, each with the mouth on the other’s genitals. If the guy is on top the girl can hold the bottom of the penis with her hand so she has some control over the movements and doesn’t feel choked.
The position works only well if you and your partner are of similar height.
Some like the simultaneous activity, while others find they have to do too many things at once. It can be hard to keep your mind on what you’re doing and your own feelings at the same time.
If you’re going down on your lover for the first time, position 69 is not advisable. You better take turns, since this makes it easier to give instructions, talking about what you like and don’t like.