You can navigate to a type declaration of a variable, field, or method under the caret. Press Ctrl + Shift + T keyboard shortcut or select ReSharper | Go To | Type Declaration menu item from the main menu.
The source file will be opened and the caret will be positioned at a declaration of a class/struct/enum that specifies the variable/field type or the type returned by the method. If the file with this item is closed it will be opened in a new tab. For library types, the corresponding item will be displayed in the Object Browser window.
Consider the following example:
After user presses Ctrl + Shift + T the Object Browser window will be opened at the declaration of the type System.Char. Notice the difference with the Go To Declaration feature behavior!