PLR = Private Label Rights. These are books, software, videos, that are re-brandable. You can give them away as incentives for people to join your mailing list. You can re-brand them and SELL them. You have to check each item to see whether it can be given away or if it must be sold. Just do some Google searches and you'll find more than you ever wanted to know about PLR.
PLR is good cuz you can adjust the creative to get better conversions. You need to push volume to get these rights though.
PLR can be a great way to get your hands on professional products that you can claim as your own. Just be sure to treat each PLR item differently, as they all have different usage policies. Matt