PgAdmin 4.1.1 question

Hi,

I am trying to find more information about the PgAdmin 4 Tabbed Browser. In the page, we have a couple of graphs and the documentation said:

The Dashboard tab provides a graphical analysis of the usage statistics for the selected server or database:

* The Server sessions or Database sessions graph displays the interactions with the managed server or database.
* The Transactions per second graph displays the commits, rollbacks, and total transactions per second that are taking place on the managed server or database.
* The Tuples In graph displays the number of tuples inserted, updated, and deleted into the managed server or database.
* The Tuples out graph displays the number of tuples fetched and returned into the managed server or database.
* The Block I/O graph displays the number of transactions processed by the managed server or database.

Is there any other documentation explain what those graphs means? More specifically, the last graph "Block I/O" does not make a whole lot of sense to me... What does Block I/O has to do with the number of transactions processed?

Ed.