Print-on-demand is a fulfilment method for producing products on a per-order basis. In other words, products are only produced once they have been purchased by a customer.
This yields several benefits to you as a creator and seller, as it means that you do not have to pay for the product until after you've sold it. This also eliminates the need for you to hold any inventory or bear inventory risk, i.e. the risk of having too much inventory and not being able to sell what you've already bought.
While PoD products do tend to come with higher production costs, it is an easy way to start selling while developing an understanding of the demand for various products or artworks. This can then help you better project sales demand, should you decide to purchase your inventory upfront.