Think of the subconscious mind as the storage room of everything that is currently not in your conscious mind.
The subconscious mind stores all of your previous life experiences, your beliefs, your memories, you skills, all situations you've been through and all images you've ever seen.
The best way to understand the subconscious mind is to look at the example of the person who wants to learn how to drive a car. At the beginning he wouldn't be able to hold a conversation with anyone while driving as he would be focusing on the different moves involved. That's because he's still using his conscious mind to drive.
Programming your subconscious can be done through hypnosis. The subconscious mind learns by repetition and not by logic. This is why you can convince someone to believe in something by repeating your argument again and again rather than using logic. For more information on this topic see the guide to the psychology of convincing.