Incremental search ad clicks are clicks which add-up to organic clicks on SERPs. They are visits driven by search ads and that are lost when ads are paused.
Incremental clicks are measured to examine the value and interest of bidding on keywords for which a company has already an organic presence due to SEO.
Incremental clicks are especially in question for branded keywords for which companies have often the first rank in organic results. It seems that incremental clicks are more important for mobile search.