To use tab completion in the Command Prompt, just enter the command and then the portion of the path that you do know, if at all. Then press the Tab key over and over to cycle through all of the available possibilities.
For example, let's say you want to change directories to some folder in the Windows directory but you're not sure what it's named.
Type cd c:windows and then press tab until you see the folder you're looking for.
The results cycle in order or you can use Shift+Tab to step through the results in reverse.
You know how your smartphone's texting app automatically guesses what it is you want to type next? Tab completion in Command Prompt is sort of like that—only better.