A simple way to think about blocks is how they apply to a large, simple oval.
You break that oval up into electrically isolated sections (blocks) so that a different power source can be used to power each block.
This then also allows you to completely cut the power to a given block. With conventional operation, that might be a siding where you park a train while another continues around the oval.
Another definition of the stop block might be where one block of the oval is electrically tethered to another block of the oval (usually on opposite sides of the oval). When train 1 reaches the "dead" block, it sits there until train 2 arrives at a block that then electrically activates the block that train 1 is sitting in.
If spaced correctly, and the train speeds and lengths are about the same, it can give the appearance of two trains chasing each other around the oval, starting and stopping automatically.
The old Lionel manuals from the 50's have this depicted in various forms.
