In the video, Kara Pernice talks about how to design a good search UI. She says that having a search function is very important, because people will want to search things on your website. However, the reason why people may not be using the search function very much on a website is because of its visibility. She suggests having a blank search bar at the top of the screen, rather than just having a magnifying glass icon, or having text in the search bar which can also take away visibility. She also points out another issue with the icon search function, saying that it requires the user to click in order to expose the search field before they can type a query. This "introduces unnecessary interaction costs." On the other hand, with mobile, depending on how important the search function is to your website, you may want to have a collapsed search function because of the lack of space.