A banister is the long bar you hold when you go up or down stairs, together with the row of posts that hold that bar up. The posts stand along the open side so you do not fall off the edge. In houses it is often wood; in some buildings it is metal. Though actually, people are a little loose with the word: some say banister for the whole rail-and-posts, and handrail for the bar you hold. You hold the banister, run your hand along it, or (children do this, though it is not safe) slide down it. It is an ordinary home-and-building word, not a fancy one. Some people spell it bannister, with two n’s; that is the same thing.