WLAN Link quality: What does it mean?

What does link quality mean in WLAN ?

It's dependent on several factors, from the Federal Standard 1037C for Telecommunications:

link quality analysis (LQA): In adaptive high-frequency (HF) radio, the overall process by which measurements of signal quality are made, assessed, and analyzed.
Note 1: In LQA, signal quality is determined by measuring, assessing, and analyzing link parameters, such as bit error ratio (BER), and the levels of the ratio of signal-plus-noise-plus-distortion to noise-plus-distortion (SINAD). Measurements are stored at--and exchanged between--stations, for use in making decisions about link establishment.
Note 2: For adaptive HF radio, LQA is automatically performed and is usually based on analyses of pseudo-BERs and SINAD readings.

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.