The phrase "to lie down" means to recline or rest in a horizontal position. It can refer to physically lying on one's back, stomach, or side, either on a bed, couch, or other surface. The action of lying down is often done when someone feels tired, needs to take a nap, or wants to relax and unwind after a long day. Additionally, it can also refer to putting something down, such as placing an object gently onto a surface.