Buddhism is enormously complex and diverse. Understanding it in terms of “yanas” helps figure out which parts of it matter for you personally.
“Yanas” are “vehicles” within Buddhism. A vehicle is a machine that takes you from one place to another.
Different vehicles are useful depending on where you are and where you want to go. If you don’t want to go where a vehicle takes you, it’s irrelevant and you can ignore it. If you aren’t where you need to be to board the vehicle, either you can ignore it as irrelevant, or you can figure out how to get there.
Vividness mainly discusses Tantrayana, one of the Buddhist yanas. It is less well-known and less-understood in the West than Sutrayana, which is roughly “traditional mainstream Buddhism.”