It is an item which cannot be sold as new. Because the box is damaged, the seals are broken, there could be missing parts, or it could be refurbished, or a returned item. You tend to have 14 days warranty with Overclockers, and the rest of the warranty is down to the manufacturer, so when you have the product you will need to contact them and tell them your serial number to see if there is any left over warranty. There tends to be at least a year most of the time.
They are great deals though, I have purchased a few B-Grade items myself.
This is a very common question asked. I am surprised that it is not listed on the website of what it means.