What do the bars mean?
That's a good question that I unfortunately do not have a straight answer to.

What you are asking is the fundamental problem with obtaining information from closed source frameworks. While I can programatically get the information, there is no description and thus no solid explanation as to what these numbers represent exactly. If I had this information I would love to put in a grid overtop of the graph so there would be a better numerical representation of the network status.

To my knowledge the bars appear to represent the link (green) and communications quality (blue) as stated in the app. :)
You can think of the link (the signal/source) quality as a persons voice. If it is soft (weak wireless card), the widget will show a low green bar and if it is loud (powerful wireless card), it would show a high green bar.
You can think of the communications strength as to how easy it is to understand what the person is saying. If they are far away, but are yelling or are directing their voice with their hands (using a directional wireless antenna) and you can hear them, then the blue bar would be high. On the contrary, if they were yelling, but in a crowd where everyone else is yelling (static/ambient wireless traffic) then you would not understand them and theÊblue bar would be low.

Hopefully sometime I'll be able to get in contact with a nice (anonymous or not) Broadcom or Apple employee and find out exactly what these values represent as well the range of said values.