What does one monitor means?

One monitor means that you can have one type of test for a single URL or server. So in case of external monitor that means you can have only one check(http, https, IMAP, POP3, etc.) on a single URL. In case you will need to monitor both http and https on a single URL you need to have 2 monitors. The same is true for internal monitors, so 1 monitor is a test of a particular type(Hard Drive, Memory, CPU utilization). In case you need to monitor all of these on a single server you will need to have 3 internal monitors available.