This quote is from TPM, when Qui-Gon speaks to Anakin about leaving Tatooine and becoming a Jedi.

This is one of my most favorite quotes of Star Wars in general because of how true it really is and how much it connects to the overarching story.

What I think Qui-Gon is saying here is that whatever you dwell on (and whatever you don't) shapes the reality around you. If you focus on the same thoughts again and again your subconscious mind will do it's best to manifest that into reality. This could be for the better or for the worse, it comes down to what you choose to focus on.

The coolest part about this though is that Obi-Wan tells Anakin the same thing, but in different words in ROTS. He told him to "mind his thoughts". 

The thing is though, without Qui-Gon acting as a father figure and the help of the Jedi, Anakin couldn't fight his demons and "mind his thoughts", because at the end of the day he was alone. This is one of the main reasons he becomes Darth Vader.