'You're' is a contraction of the words 'you are.' When using 'You're' with the words 'supposed to' you are making a suggestion that something you strongly believe ought to happen.

'You're'是'you are'的缩写形式，当你把'You're'和'supposed to'一起使用的时候，你在暗示一件你坚信应该发生的事。

Here are some examples: 这里有一些例子： 

"You're supposed to keep that secret."

 你应该保守秘密。

"You're supposed to let me know when you leave."

 你应该让我知道你什么时候离开。

"You're supposed to stop when at a red light."

 你应该在红灯的时候停下来。

"You're supposed to unpack once you get there."

 你一到那里就应该打开行李。

"You're supposed to return the movies you rent on time."

 你应该按时归还你租用的影片。

"You are supposed to remain calm."

 你应该保持冷静。

"You are supposed to fasten your seat belt."

 你应该系上安全带。

"You are supposed to invite all your friends."

 你应该邀请你所有的朋友。

"You are supposed to encourage one another." 

你们应该互相鼓励。

"You are supposed to decide before next Thursday."

 你应该在下周四之前做出决定。