Welcome aboard! A project is some set of steps or actions that produces an outcome. "Paint the house" would be a project. "Mow the lawn" would be another.
A context is a resource - person, place, or tool, most broadly - that's critical to accomplishing some of your actions.
Assume that there's a "buy brushes" action in your house-painting project. Similarly, assume there's a "buy lawn mower" action in your lawncare project. If both of those actions are assigned to the "Home Improvement Superstore" context, when you're shopping at that store you can take a look at the context and see both actions you can accomplish there. You don't need to scan through all of your projects looking for those actions - OmniFocus finds them and gives you quick access to that list.
Contexts allow you to accomplish whatever actions you can from your entire database, regardless of which projects they may be from.
If you haven't done so already, I'd suggest looking at this
white paper we have up on the website - it walks through these concepts and may be helpful.