What is .NET Standard? At one point I thought I was savvy in .net but with all these versions, like core, framework, standard,... it's all just rather confusin

What is .NET Standard? At one point I thought I was savvy in .net but with all these versions, like core, framework, standard,... it's all just rather confusing... I'd imagine that being the single biggest customer complaint just understanding versioning and naming. Which one would you use if you do a new project, no legacy requirements?