I have two separate "print on demand" websites. My Redbubble site has more choices in clothing, where as my Fine Art America site has more choices in wall art, and other things. Print on demand is simply a company that takes your order and prints whatever you want on whatever item you want. In the past, only businesses could afford to have things printed, because they had to order everything in bulk. Print on demand companies take small orders, even if it is only one item.
It is great for artists, because we can now offer our art printed onto many usable items, instead of only being confined to something that hangs on a wall. We do not have to keep an inventory of every single thing and hope that it sells. Instead we upload our high quality image to the company and they take it from there, taking a customer's order and mailing it directly to the customer. We still get a commission for our art work and own the copyrights on it.