This statement is really about whether or not your thinking is limiting or expanding your options for the future.  If your thinking today is, "The sky's the limit," there is no telling what you might accomplish tomorrow.  If your thinking today is that there really isn't all that much you can accomplish tomorrow, you are setting yourself up to not accomplish very much.  This does not necessarily mean literally the next day, but the future in general. For example, if you are thinking that you cannot get into a particular college or get a job you want, you are defeating yourself in advance, closing off all the options you really do have.  Other examples might be thinking you cannot write a good paper or take good photos.  When we are thinking about the next day, even then, we can think expansively or in a limited way.  I knew someone who would "decide" that she would be tired the next day.  Why decide that in advance?  A person could wake up and feel just fine.  If you think positively and think about keeping all your options open, there is no telling what you can do! 

The basic point of the sentence above is that the future will depend on what you are preparing yourself to do now. This makes a lot of sense. Things do not usually just happen for people, especially good things, without preparation. So, if you now think about what you will do in the future and you prepare, then you will be able to achieve much more. 

The opposite is also true. If you do nothing now, you will probably amount to nothing in the future, or do very little in the future, because you would have wasted the present.