What is "vendoring"?

Based on this answer

Defined here for Go as:

Vendoring is the act of making your own copy of the 3rd party packages your project is using. Those copies are traditionally placed inside each project and then saved in the project repository.

The context of this answer is in the Go language, but the concept still applies.

If your app depends on certain third-party code to be available you could declare a dependency and let your build system install the dependency for you.

If however the source of the third-party code is not very stable you could "vendor" that code. You take the third-party code and add it to your application in a more or less isolated way. If you take this isolation seriously you should "release" this code internally to your organization/working environment.

Another reason for vendoring is if you want to use certain third-party code but you want to change it a little bit (a fork in other words). You can copy the code, change it, release it internally and then let your build system install this piece of code.