Quality traffic can be defined when a buyer receives a high opt-in rate from a traffic vendor.
Here's an example on how to calculate quality traffic:
You buy 100 clicks from Seller A at $65 and receives an opt-in of 40%. Then, your actual cost per subscriber will be $1.625 ($65/40).
However, let's say you buy 100 clicks from Seller B at $65 and receives an opt-in of 65%. Then your actual cost per subscriber will be $1.00($65/65)
With high opt-in, your cost of investment will be lowered. Consequently, there'll be higher probability of sales. You stand a higher chance of getting more sales from 65% opt-in than 40% opt-in.
In Short: High Quality Traffic > High Optin > High Probability of Sales