used after the verb to show the action is short or to express encouragement to trying or politeness

一下 is used after a verb for emphasizing a polite request it also means once, one time, a while. for example most common is"等一下=wait a while, second"or 看一下=have a look once, or 我來 試一下=I'll try it or 按一下=press it once ( like button)

"Hello" in Chinese, there are a lot of explanation.