PPC traffic simply means ‘pay per click’ traffic. You may see some networks or programs state they allow or don’t allow PPC traffic.
An affiliate may choose to take the risk of promoting an offer via this method in order to try and earn more in commission than they spend on clicks. The Affiliate will pay an advertising network for driving them clicks to their affiliate link, they will then attempt to optimise this traffic in order to get conversions for the Advertiser. The Advertiser still pays on whichever model they are choosing to pay their affiliate program on.
The Affiliate is taking the risk to try and earn more in commission than they spend.
PPC ads are typically served on search engines (for example Google, Bing, DuckDuckGo etc) or content based websites. PPC ads on search engines typically appear at the top of search results under a title like “Sponsored links”.