https://docs.microsoft.com/en-us/sharepoint/dev/spfx/sharepoint-framework-overview
SharePoint Framework
Historically, we created web parts as full trust C# assemblies that were installed on the cloud servers. However, current development models for the most part involve JavaScript running in a browser making REST API calls to the SharePoint and Office 365 back-end workloads. C# assemblies don’t work in this world. We needed a new development model. The SharePoint Framework is the next evolution in SharePoint development.
