I think the qually bar is a measure of the amount of noise on the line , all lines of propogation have some background eletron noise plus other stuff that shouldnt be there (switching spikes etc.) the required signal has to be decoded from all the other stuff on the line , its done by using clever mathematics , checksums and things , but if the background noise is high the decoded signal gets broken and packets are lost ( My take on it anyway as an ex and long redundant television engineer )
You know such complicated stuff but don't wanna bother with gpl utilities? PLEASE!