'Let's' is a contraction for 'let us.' You are suggesting to someone that you should both agree on what you will communicate to someone else.
'Let's'是'let us'的缩写形式，你在建议某人，应该双方都同意你将要和别人交流的内容。

Here are some examples: 这里有一些例子：
 
"Let's say that you love to fish."  

比方说你喜欢钓鱼。

"Let's say we found it."  

比方说我们找到了它。

"Let's say that we enjoy being with them." 

比方说我们喜欢和他们在一起。

"Let's say that we had a good time." 

比方说我们玩的很开心。

"Let's say that it's hard to decide." 

比方说这很难做决定。

"Let's say that we have to go." 

比方说我们必须得走了。

"Let's say that we can host."  

比方说我们可以主持。

"Let's say that I have to work." 

比方说我必须得工作。

"Let's say that the movie was really good." 

比方说那个电影很棒。