F10, F12, F15, F21, etc. are definitions coined by Robert Monroe, arguable a, if not the, modern day founding father in bringing Astral Projection to a widespread and "respected" field of study.
With these "focus levels", he was attempting to create a discussable framework for discipline of astral projection. These "Foci" were his centralization for common patterns he experienced. The reason you should familiarize yourself with these numbers is that what we are discussing here is completely subjective. Any common word will have bias, better said, you will have bias to any word with context. Thus a Letter and number is a more productive concept for you to form your knowledge base around. It has high verifiability amongst peers. Each one of these "states of consciousness" have attributes surrounding them. Memorizing them and attempting to understand what Monroe was talking about will be essential in your understanding of what you are experiencing when you are alone in your own head. It's not like you can easily Google a quick question while in that mindset.
Of course I, alongside others, do not believe Monroe was 100% accurate in his depiction. Regardless, as the Lewis and Clark of this domain of science, his nomenclature should be upheld and respected as it provides a nice sequential pattern in which we can cluster main concepts around. For the lower levels of the astral, at least, I would argue that it is extremely accurate. I made a helpful conceptual framework that is Stickied on the main page in this forum. Look up top for it, it will lay out most of what he said into a nice single picture.
In addition, there is a wonderful F10 primer written by Major Tom right here.
http://www.astralpulse.com/forums/welcome_to_permanent_astral_topics/focus_10_mind_awakebody_asleep-t24783.0.htmlUnderstanding what F10 is, why it is important and practicing reaching one will be your first mission if you are serious about this. Just look at all the stickies in this site. Everything you could want to know to become an expert is right here. You hit the jackpot by coming here. Anything you don't understand just type in the search engine. Stay skeptical and form your own opinion through your own experiences.
Best on your travels,
Cheers,
Contenteo