This isn’t about periscopes or mouthwash—when it comes to searching, a scope is the area in which a search takes place. When you use the Search field in a Finder window to look for files and folders, you have the choice of two scopes: This Mac or the current folder. You can always switch the scope after starting the search by clicking the other choice near the top of the window, but it’s easier to set the default search scope in Finder > Preferences > Advanced so it’s set right to start. From the “When performing a search” pop-up menu, choose Search This Mac to search across all indexed drives, Search the Current Folder to limit the search to the folder showing when you start the search, or Use the Previous Search Scope. For most people I recommend setting it to “This Mac” so searches start more inclusively and can be narrowed down if you so choose.