Solid (or SolidJS) is an open source JavaScript library for building user interfaces, created by Ryan Carniato, which borrows React's syntax (JSX, components) but works without a virtual DOM, thanks to fine-grained reactivity based on signals.
In React, a state change re-runs the component's render. In Solid, each component runs only once: only the parts of the page that depend on a changed signal are updated. The result: very good performance and very little code sent to the browser.
SolidStart plays for Solid the role Next.js plays for React. The community remains much smaller than React's, but Solid helped popularize signals, an approach since adopted in various forms by Angular, Svelte, Preact and Vue.
Why it matters when hiring
Few companies hire specifically for Solid: searching on that keyword alone sharply shrinks the talent pool. A good React developer picks up Solid quickly, as long as they understand the different reactivity model. In interviews, ask them to explain why a Solid component does not re-run: that is the test that shows whether they understood the tool or just copied the syntax.