Notice: the following explanations about each different licensing terms may not exhaustive as product creator can release their digital products with slightly different licensing terms attached to them. You may have to download the digital product to view the complete license details to abide by the creator's wishes. Followings would be the most common license definitions that most of PLR & MRR digital product creators agree on.
PLR License (The term 'PLR' stands for Private Label Rights.)
Every PLR license may include slightly different license terms. But basically, the license "Private Label Rights" allows you to edit, change or label/rebrand the product contents in any way that you see fit. You can put your name as the author of the digital product (you may need to do some significant changes before you can claim to be the author of that digital product. You could change some words here and there or do a total change of the digital product) You’ll get a "non-exclusive license" which means others also can license the same item, just like stock images. Most favorably, unlike other digital contents, the PLR contents have no ongoing license fees or royalties with any kind of attribution to the original content producer. You may use the PLR contents to make your own courses, articles, workshops, blogs, webinars, social posts, autoresponder emails, lead magnets, or even physical products. It means you can use them in any areas of your business.
MRR License (The term 'MRR' stands for Master Resell/Resale Rights.)
The non-exclusive license 'Master Resell Rights' allows you to resell the product usually with fewer privileges. Such as, most of MRR digital products can be resold, the license passed on to the buyers, but commonly with a MRR digital product, the buyer cannot edit, change or rebrand it under his/her name. You can sell a MRR digital product by transferring the MRR license to your buyers as well. This will make your buyers so happy because they also can start a digital product business with it.
RR License (The term 'RR' stands for Resell/Resale Rights.)
The non-exclusive license 'Resell Rights' allows you to resell the product usually with more fewer privileges compared to MRR. You can resell the RR digital products but you cannot grant any further resell right license to your buyers. In other words, you can only sell the RR digital products with personal usage license to your buyers. (Your buyers cannot resell them)
Personal Use Rights
Personal Use Rights means the content or such a product cannot be resold in anyway. The content is basically for your own learning and personal use. If you cannot find any of above reseller licenses to a product, then it often means the product offers personal use only.