mysqltuner reports that I have "basic passwords in the list". What does that mean?

I've been running mysqltuner on one of my VPS's as it hosts two Magento websites, and we do tend to have fairly heavy traffic. So I wanted to start to improve the performance of MySQL as well as other parts of the store.

After 6 days of mysqltuner running, I ran the report this morning. Some very useful bits, but this one grabbed my attention:

> [–] There are 612 basic passwords in the list.

At the bottom where it gives recommendations, the passwords aren't mentioned. But the fact it's alterted me to "basic passwords" worries me. But what does this actually mean though?