Stores the exact versions of the packages installed. When you want to install these same exact versions again, you can use the package-lock.json.
To install these exact versions, use the command npm ci instead of npm install.
Example
You have express version ^4.15.3 in your package.json file and 4.15.3 was the version that was installed when you initially ran npm install, and that is what is stored in your package-lock.json file. Now there is also express version 4.25.6 available.
If you run npm install then express version 4.25.6 will be installed since the semantic version ^4.15.3 says "install the latest minor and patch release of major version 4". The same thing happens for all other dependencies in the package.json file. The package-lock.json file is then updated with the exact versions of the dependencies installed.
If you run npm ci then express version 4.15.3 will be installed, along with the exact versions of any other libraries that were installed when generating the package-lock.json file. The package-lock.json file will not change.