It means including a library in your codebase via manually copying the files in (but generally in their own subdirectory and without modifying them so that it i

It means including a library in your codebase via manually copying the files in (but generally in their own subdirectory and without modifying them so that it is easy-ish to update to a new version of the library by simply copying in the files from the new version). I believe the name comes from a tradition of putting such code in a directory named vendor/name-of-library or vendor/name-of-vendor/name-of-library which allowed for distinguishing between the src directory which was first-party code and the vendor directory which contained code form 3rd party vendors (often such code was proprietary and had to be bought/licensed). Nowadays the term is used to differentiate between a manual approach and the use of a package manager.