Vendor Trash

Items in your backpack are color-coded into 5 categories: grey, white, green, blue, and purple. Grey items are considered "vendor trash." Most of them have no use in the game. Some can be equipped as weapons and armor, but their stats will be so low, the only time you will use them is if you have no item yet in that slot and cannot afford to buy one. There are addons that will automatically sell all grey items whenever you open a vendor window, and others that will suggest grey items to throw away if you are looting a corpse and have no room left in your bags.  Get rid of the stuff.

Sometimes there are white items that used to be useful but no longer are, yet Blizzard hasn't gotten around to changing their status.  And many white items are not really valuable to anyone else so you won't be able to sell them at the Auction House.   White armor and weapons, food and drink that does not have any buff stats, things that can be bought at almost any vendor and are now below your level.  All that should be considered vendor trash and sold for whatever you can get.