М is for Mempool.

What it is

The mempool is the waiting room of the chain. A new transaction sits there until a miner includes it in a block.

Each node keeps its own copy, so there is no single official mempool. What you see is one honest view of the queue.

How to read it

A swelling mempool means demand for block space is rising, and fees tend to follow. It is the closest thing the chain has to a queue outside the door.