AdSense paid traffic refers usually to very low quality traffic bought at very low price by a site owner to generate revenue on his website through AdSense. Of course, the goal is to generate higher AdSense revenue than the cost of acquiring traffic.
For having a chance to success, the traffic must be AdSense safe and the CPC and CTR high enough.
Sometimes paid traffic is used together with AdSense clickjacking for ensuring sufficient CTR.
AdSense paid traffic strongly differs from traffic coming from classic means of buying traffic for ecommerce sites.
An example of ad for AdSense paid traffic: