What does this mean a trundle bed a low bed on casters?

I have an antique bed that sits high off the floor. I have 2 wooden steps to get up onto it. Under it I have a trundle bed. It's on castors (little wheels) so I can slide it out. If I have my kids come over. I just pull out the trundle bed and it's almost as big as a double bed. They are comfy and convenient.