This article explains what each type of connector in your preferences menu under the "Connectors" tab does for your ReadCube client. ReadCube uses data from several search engines to help you find new source material and to match your articles when searching and importing with appropriate metadata. You can use this settings menu to limit or expand what search engines ReadCube has access while operating.
Search Connectors: These are the search engines that appear when you search for new content in ReadCube. If multiple search engines are turned on, you will be able to select from Google Scholar, Pubmed, or Microsoft Academic from the top of you ReadCube search window. If you do not want any search engines to appear, simply uncheck its box and it will no longer be available for use until you check it again.
Full Text Sources: Some libraries (and individual users, who may be you!) gain access to their research content through aggregator subscriptions. Essentially, these subscriptions have specific deals with publishers to access various content at group rates. As of a few months ago, ReadCube has implemented support to integrate these subscriptions into your desktop client. If you or your library have one of these, simply select the appropriate box and your ReadCube should be effectively configured.