Not many houses these days have a moat — it's a trench dug outside a castle and is usually full of water. A moat means "Keep out!"
You're most likely to hear about moats if you read a fairy tale or a history of medieval times. This water-filled ditch is one of the lines of defense for a castle. In the old days, it was pretty effective. If you filled the moat with a deadly fish — like a piranha — that was an even better defense. A moat is pretty much the opposite of a welcome mat.
