The document in lesson 10 is there to show you the chords for a standard 12 bar blues. So those letters, G, C and D are there in reference to guitar chords or the ‘backing’ for your harmonica. The main thing to note is that each chord is referenced by a number in my course. So G is 1. C is 4 and D is 5. This makes it much easier for me to teach licks that go over certain sections of the 12 bar blues as the course progresses.
I think your question relates to the task I give you at the end of the video lesson. All I ask there, is that you play the second position blues scale up and down over the provided jam track. This should be with clean and clear single notes. Each note can be any length you like. You may even have some ideas of your own as to rhythm. Chords, and tongue blocking applications are taught later in the course.
To answer your other question. A 12 bar blues can be in any time signature at all, 3/4 and 6/8 are definitely used. Having said that 90% of blues you’ll find will be in 4/4.
Hope that helps!?