A course is a container for all your content that will be re-used every time you plan an offering.
An offering is a specific instance of the course being offered on a specific date, in a specific location, with a specific speaker, etc.
Think of a course like a recipe for baking a cake. The recipe contains all the instructions and ingredients needed to make the cake. Now, imagine events as the actual times when people gather to bake the cake using that recipe. So, while there's just one recipe (course), you can have multiple baking sessions (events) where people use that same recipe to bake cakes. Each baking session might happen at different times or locations, but they all follow the same instructions from the recipe (course).