Green is good, yellow is marginal in the above meter. The minuses reflect a loss or reduction in power. If I remember correctly, the meeting room at the Prescott Library, where we hold the PMUG meetings, has an RSSI of about a -70 and we usually can connect okay. As I'm typing this I'm close to the router and my RSSI is a -40. In my experience, if you show an RSSI of around -80 you probably won't be able to connect to the internet.
In addition to strength, another important element of a WiFi signal is the amount of noise (interference from microwave, cell phones, etc) present. If you're showing a good WiFi signal but are having difficulty connecting, or staying connected, to the internet, then possibly you've got noise interference. A spread of 15-20 dBm is desirable. For example, say the strength is -60 and the noise is -80, you're in good shape.
What can you do to improve RSSI? Move closer to the base station or router; walk around and change the location of your computer -- maybe you'll move from behind an obstruction; buy a WiFi booster. I've used several, and they do help.
I've got some friends that recently converted to a Mac and I thought this might be helpful for them. The same concept applies to PC users, but I don't recall (if I ever knew) how to measure RSSI on a PC."