Used to strengthen slopes of soft soil and prevent erosion, in places such as river and harbor banks, roadway embankments, and so on, piled with or without mortar. As in, that's what that's called. Can also be used as a verb, to build with or strengthen with stones. The origin is obscure, but it's apparently a reduplicative form of rap in the sense of a blow/strike, and it's been around since the 1570s.
The children scrambled down the riprap to the lake water.
(To readers in the States, have a safe and happy Thanksgiving, whether it be on or away from a shoreline.)