Dependency
aka: packageaka: npmaka: node_modules
The Quick Version
Code written by someone else that your project uses.
How it actually works
Dependencies are packages listed in package.json and downloaded into node_modules. Your build fails if a dependency is missing, and your site can break if one updates in an incompatible way.
Why does it matter?
'Cannot find module x' during a build almost always means a dependency was never installed or never committed to package.json.
Example
"react": "^19.0.0" in package.json