AdSense safe traffic is a term commonly used to refer to traffic which is not supposed to be considered as invalid or incorrect clicks when turned into clicks on an AdSense website.
AdSense safe is a term used to qualify AdSense paid traffic.
A traffic is usually considered as AdSense safe when:
it is human traffic
it is US traffic (for US Adsense partners)
it doesn’t present statistical anomalies
Of course, most of low price AdSense paid traffic is not really AdSense safe - average north american doesn’t visit a site for a fraction of a cent - and AdSense paid traffic world is far to be a clear world.