In the world of software development there are ‘libraries’ which perform specific jobs. For example there are libraries that allow you to manipulate time, to figure out things like timezones and durations, which can all get quite nasty otherwise. The nice thing about a library is that it is made in one place, and it can be used everywhere. Every piece of software you ever interact with will use a time library. It’s like a specific part for a car radiator, made in a factory somewhere and then put into every car that’s ever made. Car companies don’t need to worry about that part: it’s just there.