A doorskip involves going Out of Bounds by going past the door transition blocks.
This is sometimes done by accident during 100% runs during the Gauntlet Spark from Landing Site, by pausing on a specific frame.
This is also done by pausing on a specific frame before the door transition at the bottom of Parlor, and then expanding Samus' hitbox as you pass the transition blocks. One setup for this is shown in the route video linked from 3%, another setup is shown in this tutorial for the current fastest route for Any% Glitched.
In the notes for this TAS, Total gave the backstory of this trick's discovery:
One of the major glitches that makes this all possible was something discovered by Cpadolf a while ago. He noticed that during the last frame of unpausing, when the game changes game mode, it will not trigger door transitions. This made it possible for example to pause and time a shinespark to fly through door transitions where there was no solid blocks behind. Later on Sniq found out that by pausing over and over and moving one frame at the time with the morph ball it is possible to slowly pass through door transitions without triggering them and then falling OoB. This made a lot of interesting things possible, like being able to skip bombs completely in favour of power bombs using OoB movement and a lot of other interesting things. After some more research it was also found that it is possible to jump up through doors if you can get close enough to the door without triggering it, for example by standing on an enemy close to the door. So finally me and Sniq figured that if it's possible to go up through doors, of course it has to be possible to also go down through doors. This turned out to be true and by using some precise hitbox manipulation it's possible to just fall right through.