When a site becomes slow, the database is where to look first and the hardest thing to look at quickly. This page shows uptime, current and maximum connections, slow query count, cache behaviour and the size of each database — enough to tell within a few seconds whether the problem lives here.
Connections against the configured maximum is the number that explains most outages. A server that has hit its limit refuses new connections, and the application reports a database error that says nothing about the real cause. Slow queries are the other end: a handful is normal, a count climbing steadily usually means a query lost its index during a schema change.
Database sizes are shown because growth is easier to act on early. A table growing without bound — session rows, a log table, a queue nothing consumes — is a slow problem while there is free space and an urgent one at three in the morning when there is not. The disk page in this panel shows the other side of that.
It is worth knowing what this page sees and what it does not. It reads the state of MySQL from the inside — connections, queries, sizes — but says nothing about what the database stands on. A stopped service is what Monit notices, and a drive that has started to fail is what SMART warns about, usually well before the slowdown shows up in queries. The cause of a slow database often lies below the database itself.