The most basic action you can perform on an object is to open it. "Open" is a very general term, and the action taken depends on the type of object you are opening. For example, opening a folder changes the File Manager view to show the contents of that folder. Opening a data file usually starts the application that created the file and loads the data file.
Double-click the icon.
Double-clicking an icon executes the object's default action, which is always the first action in the Actions portion of the Selected menu. For most data files, the default action opens the file by starting the appropriate application and loading the file.
Or, select the icon, then go to the menu bar and choose Open (or Open In Place or Open New View) from the Selected menu.
Or, choose an Open item from the icon's pop-up menu (displayed by pressing Shift+F10 or mouse button 3).