Contextual Link Inventory or CLI is text-link advertising that is displayed based on the relevance of surrounding content.
Contextual Link Inventory is mostly done on websites (via networks like Google Adsense) and via search engine results pages (SERPS).
The ad server checks the content of pages or search queries (on SERPS) to find matching ads. The success of the Contextual Link Inventory depends on the quality of the content and the relevance of the content to the niche/theme or the core objectives of the web site. Ads that connect better to the pages content have higher click-trough rates then unrelated ads.