Library
A package of prewritten code a developer pulls into a project to handle one job, like charts, payments, or sign-in.
What it tells you about a developer
Authoring a used library is a stronger signal than using one. A candidate whose package has real downloads or dependents has shipped code that other developers trusted enough to build on.
A typical application uses dozens of libraries. Developers publish libraries too. A library that other people download and depend on is strong evidence of quality, because strangers chose to build on that code. A framework is bigger and structures a whole application, while a library does one job.