I got some experience in this. Generally Refurbished means that its a product that was sold, and opened by a costumer. But for some reason returned. A small percentage of these have been broken but fixed. Generally its just returns. Keep this in mind when you see the price.
That's more relevant for ordinary mass consumption stuff like all sorts of electronic gadgets people buy from amazon etc. A lot of those get returned and then re-sold as "refurbished", so they're often new or like new items. But with something like server PSUs it's rarely the case, most of the refurbished server hardware was used before, sometimes used a lot. Then it gets decommissioned, and either goes straight to some sale platform like ebay and gets listed as "used", or it first goes through some technicians that clean it, test it, and then list it for sale as "refurbished".
So as far as I'm aware most of the refurbished server hardware was used before. Often used in production systems for years. Still worth buying if the price is right, but don't expect it to be like new.
