‘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.
Here are some examples:
“Let’s say that you love to fish.”
“Let’s say we found it.”
“Let’s say that we enjoy being with them.”
“Let’s say that we had a good time.”
“Let’s say that it’s hard to decide.”
“Let’s say that we have to go.”
“Let’s say that we can host.”
“Let’s say that I have to work.”
“Let’s say that the movie was really good.“