PLR - Private Label Rights, RR - Resell rights, MRR - Master Resale Rights
Digital products (e.g. eBooks, articles etc.) are often sold with different rights that determine how the buyer can use the product.
PLR (Private Label Rights) allow the buyer to use the product any way they choose.
RR (Resell rights) allow you to resell the product to others.
MRR (Master Resale Rights) allow you to resell not only the product but also the rights to others to resell the product.