This is an excellent resource here from Jonny Elwyn. Understanding the FCPx magnetic timeline. for editing video.,
It starts from the basics and has a comprehensive writeup and video tutorials.
"Before we go much further it makes sense to translate a bit of FCPX terminology:
Library = Project | Event = Bin | Project = Sequence | Primary Storyline = Track 1 | Secondary Storyline = Other Tracks/Group of Clips | Roles = Tracks if you had stayed organised* | Compound Clips = Nested Group of Clips |"