DE is for "Desktop environment". Most distro let you choose from quite a few. The two best known one are Gnome and KDE, i believe. But there are many more.
"Desktop environment" might just be two words without much meaning to you. If that the case, in the linux context, a desktop environment is basically the GUI to your computer: file explorer, desktop, taskbar, general setting manager and so on.
No, a car seat is not a good choice for an office chair, unless your office is in a racing car.
Get a real office chair, something without the stupid markup that comes with the 'gaming' moniker. If you can afford it consider brands such as Herman-Miller and the like. With just a bit of luck you might be able to find used one for a reasonable price.