Related keywords are keywords which are automatically selected to extend the list of keywords set by the advertiser. By using related keywords your campaign is topped up with new keywords and as a result you can increase your target audience and waste less time on selecting keywords.
Ads for these selected keywords are served on Yandex search results pages and search sites belonging to the Yandex Advertising Network.
Related keywords are enabled by default for new campaigns. You can disable them on the campaign settings page in the Keyword optimization section.