Engineered Hardwood is real wood comprised of layers of wood, like plywood, to be more stable. Wood only swells in one direction and the layers work against each other preventing the wood to move (or cup or buckle).
Engineered Hardwood has 3-7 layers of cross grain so you don’t get as much contraction and expansion in the wood, making it more stable. Therefore, the wood can be as strong as solid wood but is thinner.
Engineered wood can be installed below the ground level, so it is a great choice for finished basements.