There are several types of libraries that you can use for your scripting needs:
Pre-installed libraries are shipped with ReadyAPI. Some of them can be used in scripting editors right away; some of them should be imported first.
Sometimes, the functionality provided by pre-installed libraries might be insufficient. In this case, you can extend ReadyAPI with external Java libraries, either compiled by yourself or provided by a third party.
If you use the same Groovy scripts in multiple projects, you can store them in Groovy script libraries and reuse them easily.