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Variance, as a general term, means the amount of variation from a nominal value that you can expect from a given system.
For example, the postman may generally deliver the mail to your home at 10am. However, depending on the day, he can arrive anywhere from 9:45 am to 10:15 am. The variance for his arrival time would be +/- 15 minutes. In a wireless system, many things could vary, and therefore have a level of variance. Things that could vary might be (as you mentioned) signal power, but also signal delay, carrier frequency shift (think Doppler Effect, if the source is moving relative to the receiver), carrier frequency drift (over temperature/time, etc).
With many variables that could move around, you need to set a limit on how much you can allow them to change, such that your system can compensate for them. Once the variations exceed those preset thresholds, then your system is not guaranteed to function (the more robust a system is, the more difficult it is to design and build, and therefore costs more).
Those are a few things to get you thinking about all the things that could vary in a wireless system, and why you'd want to account for them.