A monorepo is a single code repository that holds several distinct projects (applications, services, shared libraries), as opposed to a polyrepo where each project has its own repository.
The benefits: changing code used in several places in one go, easily sharing code and configuration, having an overall view. The trade-offs: a large repository and build tools that must understand internal dependencies so they only rebuild and retest what changed.
In JavaScript and TypeScript, teams use Turborepo, Nx or pnpm and Yarn workspaces. Bazel and Pants cover multi-language monorepos. Google and Meta are the best-known examples of very large-scale monorepos.
Why it matters when hiring
A monorepo is an organizational choice more than a rare skill. It matters when you hire someone for tooling (Platform Engineer, Staff Engineer): ask how they kept CI fast as the repository grew. For a product developer, having worked in one is enough; it is not a deal-breaker.