From FDT Documentation

Default key binding: F3 While writing the code it is often necessary to change between different classes and other declarations. In order to expedite the work FDT defines some hot keys making it possible to go to declarations very quickly. The most important hot key is F3. If the cursor is placed on a literal - no matter, if it is a variable, a function or a class - using F3 makes the cursor jump directly to the declaration. If the cursor is placed on a literal and F4 is used, you get to the type declaration. In case of a variable it is the variable's type, in case of a function it is the type of a return value. F5 only works with functions that either overwrite other functions or that implement an interface. Here using F5 leads you to the overwritten method or the declaration within the interface respectively.