To lie down in English means to recline or to rest in a horizontal position. This phrasing comes from the verb "to lie," which means "to be in a horizontal position" or "to rest." Here are some examples of how you might use this phrase in a sentence:

I need to lie down and take a nap. (我需要躺下小睡一会。)

After a long day at work, I just want to lie down on the couch and relax. (工作了一整天后，我只想躺在沙发上放松一下。)

The patient was instructed to lie down on the examination table. (病人被指示躺在检查台上。)