A library is a tool which performs specific, well-defined operations. A front-end developer can call operations from a library in their own code when necessary. During development, front-end developers use libraries to make complex operation simple or to prevent reinventing the wheel for common funcionalities. En example of a library is D3.js, which is used for datavisualisation.
Libraries differ from frameworks. Both are used to make development more efficient. But where a library provides tools a developer can integrate in their own application setup, a framework provides a skeleton wherein the developer needs to develop. In other words, a framework is more opinionated than a library.