Stencil is a compiler that generates Custom Elements (part of the Web Components specification) for reuse in any JavaScript web framework.
Stencil was conceived by the creators of the Ionic Framework as means to build reliable framework-agnostic components. The Stencil developers put emphasis on creating progressive web apps that make use of modern browser features in a user-friendly manner.
So, no matter if you use Angular, React or Vue.js for developing web applications Stencil allows you create and share reusable components that run everywhere.