The gold pellet rating is for the best of the best, a great pellet. Great BTU output, clean burning with little to no carbon build up, very few fines, and consistent sizes. These are pellets that should burn well in just about any stove. This rating is to basically indicate the best pellets available on the market, with no significant flaws.
The silver pellet is a good pellet that has a some flaws that keep it from being a gold pellet. For instance I just tested a pellet that was close to being a gold, but the pellet sizes were wildly different. The silver pellet is still a step above the bronze, and performs well in more areas than it performs poorly in.
The bronze pellet is the pellet that gets a PFI rating of premium (just like the gold and silver), but is just a generally low grade pellet. It is not to say these pellets will not work for you, but they likely have a number of significant issues. Lower BTU output, high ash production, significant carbon build up, high fines content, and so forth. It is not to say a bronze pellet will have all of those issues, but just to say they do more things poorly than they do well. It is also not to say that they will not burn well in your stove, but if you do get them make sure they are cheap.
Reviews not done by me will not receive this rating.