I think that's understandable.
Try it this way - going backward up the production line ;-)
You have exported something into Youtube - that's now a Movie (go to Theater - top middle of window- to see these)
It was a series of changes you'd made to clips - that's called a Project (Go to Projects - top middle of window- to see these)
To create the project you pulled in bits of footage - these are Events (Go to Media -top middle of window- or click on Library in the left hand nav column to see these).
So let's go the other way. Imagine you're making a film about a ski trip - no let's do a beach trip - that you shot.
You have footage of 1- getting out of the car 2-putting on wetsuits 3- getting on your boards 4 - catching a nice wave
You upload those into media and name them individually (these names will appear above the preview window if you keep "I" for information button turned on - AND in the left hand nav column in Library as Events.
Those four things are Events in the eyes of Mac. They are also referred to as clips.
You hit "New Project" and begin to pull part or all of these bits into your Timeline and start modifying them. When you go to leave - IMovie will ask you to give it a name. This is your Project i.e. modified bits of events that are not yet exported into a movie.
Once you've tweaked as much as you want - you've added music and some sweet ending - you "export" into your favorite format or platform - now you've made a Movie and the Project no longer exists. (But the Events remain).
To see the events go to Library or hit Media at the top, to see the projects you're working on click on Projects at the top, to see exhorted projects click on Theater at the top.
PS The raw events will stay as Events. But if you think you will want to come back and re-use the edited Events simply make a duplicate of the project then only export one. Do this by clicking on the ... thingy underneath the Project. The other will stay in your Library.