PoD stands for "Print on Demand". You will sometimes hear it referred to as "On Demand Printing". It describes the technology that enables book producers to print one copy of a book at a time instead of in large runs. Traditional offset printers will run off hundreds or thousands of copies of your book at a time. With PoD printing a book can be printed one copy at a time. This allows your book to only have a copy printed when someone places an order for your book or to order very small runs. This makes having your book printed affordable as you are not paying for hundreds or thousands of copies of your book as well as having to warehouse them all.
There are some differences in quality between a book that was printed using the traditional offset method and a book printed using the print on demand method. PoD books usually have a different stock of paper than offset printed books and the cover printing method is also different. However the PoD method still produces a good quality book and most people will not notice or care about the difference.
Some subsidy publishing companies refer to themselves as a "PoD publisher" or their services as "On Demand Publishing" what they are referring to is what method they use to have your book printed, not what kind of publishing services they provide.