If Carla click on a webpage form a SERP and then immediately hits the "back" button, search engines would interpret this action as meaning the

If Carla click on a webpage form a SERP and then immediately hits the "back" button, search engines would interpret this action as meaning the webpage was not relevant to her and will, therefor, reduce the webpage's rank on future search.

If Andy sees a search text ad but rather than click on the ad he types the URL into the address bar directly, then the advertiser is charged a reduced rate for the add instead of the full ad rate.