My (very simple) interpretation is it is a confining container:
Think of a Shipping Carton (case) of Energy bars as the <body>.
Inside the
case are display packs (24 bars) <p>
Inside the Display pack (usually what you see on the candy counter) a wrapped bars <span>
The bars are what you consume < the authors words >
Each block confine its contents:
A normal block stacks the contents wit the adjacent block
An Inline block places the block alongside
I also believe a block contains properties (prevents adjacent blocks from inheritance)
I would leave the "block: display;" in place
Oh!
Block also permits different controls depending on recognized device family:
Display
Print
Aural ...