KDE Plasma is an desktop environment.
The kind of thing you interact outside of installed app/programs. Like the panels, window decorations (titles, close buttom, maximalize button), the way windows float and behave, system settings, etc.
Unix systems (like Linux) are very modular and you can install different desktop environments if you want. And even within those desktops are modules, like you can install different “start menu” or file manager on KDE Plasma.