I'd love to Google this (I've tried) but I'm not sure what it is called, let alone how to do it.
Consider:
project A
project B
I am working on project A, that happens to need project B. How, do I without checking out project B with project A, have a reference to it?
In other words, there is a way (I experienced it on placement) that allows you to use a project within a project and if someone was to update that project you'd get the changes. Basically it means you don't have to worry about falling out of date or managing two copies of something.
No need (but if you can/want) explain, just a link or name will sort myself out.
Cheers.