What does link quality mean in WLAN ?
It's dependent on several factors, from the Federal Standard 1037C for Telecommunications:
Bit error rate (BER): In digital transmission, the number of bit errors is the number of received bits of a data stream over a communication channel that have been altered due to noise, interference, distortion or bit synchronization errors.
Signal-to-noise and distortion ratio (SINAD): Is calculated with power output/input from the radio waves.
I think there are a few types of link quality, but in simple terms, it means how much of the data you send and receive will make it to the destination in good condition.
High link quality means the requests you send will make it there intact, and the responses should be in good condition as well. Low quality will give you oddities. You may see corrupt images (twisted strangely, funny colors, etc), random text, or the pages might not load at all. Obviously you want to get the highest quality you can, as low quality can slow down or break your internet.