If I buy this bed, would I need anything else for it?

Rails are normally the metal rails that extend from the headboard to the footboard, they hold the bed together and you put the box spring on the rails, then the mattress on the box spring. But you should call Penny's customer service and check for sure that you don't need any other supports under the boxspring, like a frame, which is a possibility as their "rails" may only be to hold the bed together, not support the box/matt.