Subsites are the individual areas of the website that web editors have control over. The easiest way to think of a subsite is that it is a small, autonomous website that lives on top of the create WordPress installation. Most often, a subsite will be specific to one part of the college's work.
So http://mlc-wels.edu/library will be a subsite. So is http://mlc-wels.edu/network-serivces or http://mlc-wels.edu/graduate-studies. Someone who has editing privileges for the Library subsite does not necessarily have privileges for the Graduate Studies subsite.
They help us logically separate areas of the website along with providing a framework for permissions.
When creating your subsite, start with the least amount of design and then work forward. This means start with:
WordPress is really great at those two things. From there, work on the text and images you are using. After that is complete, continue to work on the text. When you are finally really REALLY happy with your text, have someone else take a look at it. When that is all said and done, it might be time to look at adding some visually-interesting widgets or color.
Remember, less is almost always more. Doing the least amount of work is preferable than starting with too much. Look for ways to clearly communicate using text and images first and only after that has failed (or you are EXTREMELY happy with what you have) is it time to look into what you might be able to do with the Advanced Editor.