Rack refers to one thousand dollars. Though that's the standard definition for rack, lot of people with a lot less money than that, usually say they have racks meaning they have lot of money. The plural of rack is racks, if an individual say they have 5 racks, it means they have 5 thousand dollars. In YC's 'Racks' song, he said "Gotta know I keep them racks - I stay counting them stacks - Them girls won't leave me alone". Rack is similar to term stack.