The L sit is an advanced abdominal exercise with a gymnastics style feel to it. It mainly works the lower abs and hip flexors. The exercise is meant to be an isometric contraction which means the pose is held as long as possible. That’s pretty difficult so to build up strength first, you can do reps and sets of raising your legs into an L position.
To do an L sit climb onto a dip bar facing the opening, so that your legs have room to swing up. Hold on to the dip bars with straight arms. Then you want to raise your legs up until they’re parallel to the ground. If you can hold an isometric contraction, keep it up for as long as you can. You can also do reps of leg raises into the L sit position. Even when you’re only doing the leg raise, try to hold the contraction for a second or two before releasing your legs back down.
If you want to add a degree of difficulty to the exercise you can also do this on gymnastics rings instead of a dip bar. The rings make it more difficult because they add an element of instability. The problem with using rings is that they can only be found on a consistent basis in Crossfit gyms. The standard commercial gym usually won’t have this tool. Dip bars are much more common in literally every setting.