"Kw," usually written with "w" as a subscript, is the equilibrium constant for water. This expression is also known as the dissociation constant or the ionisation constant.
Because of the composition of water molecules, they can act as either a base or an acid. A water molecule acting as a base can accept a hydrogen ion from a water molecule acting as an acid. When this happens, one ion each of hydroxonium and hydroxide are formed. These two ions then balance each other out. In water, this type of reaction can happen at a constant rate. Therefore, "Kw" is used to express the fact that water is at equilibrium.