Tailor Archive

Sorry if this is an old/common/newb question, but what exactly are sockets? I just bought aluxurious gownwith 2 sockets available, and when I try to equip the dress I'm reduced to my undies and told that 'This container is full. This slot is already occupied'. I'm guessing it has something to do with the sockets available, but I'm not sure. Can anyone help?

~Bottan