As soon as a message gets thru, this is reset to zero. When its non-zero, the name of the server displays in red, so you can't miss it. Reliability is the percentage of messages that worked. A high-90s percentage probably isn't a problem. No matter what, it's giving you a good idea of what percentage of user requests to the server are actually happening.
Net bandwidth contributes to a high number, as does load on the server.
Last error message is sticky. This gives you some way of having a clue as to what's going wrong on the server in question, even if you weren't watching when the error happened. Tips Run this on a machine that you know stays up. If you have enough bandwidth on your desktop machine, that's a good place to run it.