Б is for Block.

What it is

A block is one page of the chain: a batch of transactions, sealed and linked to the page before it. On Bitcoin a new one arrives about every ten minutes.

To change an old page, you would have to rewrite every page written after it. That is what makes the record so hard to alter.

How to read it

Block height is a count of pages since the first one. It tells you how far the story has come, and it sets the schedule for events such as the halving, which arrives every 210,000 blocks.