Word
Dependency
A dependency is a piece of software somebody else wrote that your project uses, so you do not have to build it yourself.
Also called: package, library
Formatting dates, talking to your store, serving web pages: solved problems. Your project keeps a list of the ones it needs, and one command fetches them all, why a project folder gets large and why there is an install step at all.
Two things to hold on to. Those pieces have their own updates and their own occasional breakages, so a project that worked last month can fail today without you touching it. And each one is code you are trusting: adding a dependency to save ten minutes is a trade, not a freebie.