**Package lock JSON and lock files are used to pin down versions of dependencies for a project, ensuring reproducible builds and avoiding potential issues with different versions being installed. Lock files serve as a rich manifest of dependencies, including metadata such as version numbers, integrity hashes, and registry locations. Both npm's `package-lock.json` and Yarn's `_yarn.lock` are used to manage dependencies, but they differ in how they handle transient dependencies and shrinkwrap files. Using lock files ensures that installations remain identical and reproducible throughout the entire dependency tree, across users and systems.