What is Blazor? It’s a framework for browser-based (client-side) applications written in .NET, running under WebAssembly. It gives you all the benefits of a rich, modern single-page application (SPA) platform while letting you use .NET end-to-end, including sharing code across server and client. The announcement post covers more about the intended use cases, timescales, and so on.
Source: Blazor: a technical introduction