DotNET(.NET), is an open source developer platform. Similar to NodeJS, this is the place where you'll install languages and libraries to be used within projects. Unlike NodeJS, .NET is actually multi-threaded. Meaning that many actions can be performed at the same time, unlike NodeJS, which is single threaded. There are three .NET implementations, they each have different uses; .NET Core will be the runtime environment used for Blazor.
Razor builds upon ASP.NET, and allows us to use C# for full-stack development. It's important to note that Razor does not follow MVC for development, and it's very unopinionated. Razor components are capable of having individual state, even if it's the same component being used multiple times.
Most simply put, Web Assembly is a way of writing code for your browser using Web Standards in a form of bytecode, similar to an assembly language, that your browser is capable of understanding. The role of Web Assembly here, is that it takes our compiled code as bytecode, and can execute this code in this browser "sandbox". Side note, this isn't actually the same bytecode that runs on your computer, it's completely separate and is only understandable by the browser. This is something that really plays a key role in expanding the playing field for languages like C#, C++, Go, and RUST to get their feet wet in web development.
In order to compile down to Web Assembly code, we need something to handle our IL (Intermediate Language code). This is done by Mono. There's nothing too crazy to worry about here, this is available to any of the .NET runtime environments. In the client of a project, you'll see a mono.wasm, this allows us to load .NET assemblies inside of our browser.
This is a load of new information for someone that's never used any of this before, so take feel free to do a bit of more research on these, here's a video tutorial of Blazor if you're more of a visual learner. Most of these technologies are explained here as well. Next week I'll work on an actual Blazor Project, so stick around!