What is .NET actually?

I apologize for a really dumb question that seems like one google search away, but i want a bit more colloquial explaination.

What is .Net really? Can someone explain it in terms like 'its like x but for y'. I have worked in IT for a long time, and i am not a beginner at all but somehow i never got to work with .NET and it seems like everyone i interact with at work used it at some point.

edit: thanks everyone for all the answers, i think i understand it now. Or atleast a little bit lmao, it seems like a huge ecosystem.