When you're discussing a topic, sometimes you have to imagine a situation and then describe what would happen in that situation. You can do this with the phrase "Let's say that..." Here's an example:
Let's say that I hire you at my company and you do a bad job. Should I have to keep paying you, or should I be allowed to fire you and hire someone better?
In this example, the speaker isn't really hiring the other person. In fact, the speaker might not even have a company! The speaker and listener are just imagining what would happen in that situation.
Here are some more examples:
Let's say that someone breaks into your house and tries to attack you. What would you do?
Let's say you ask her out and she says "no". So what? It's not the end of the world.
This expression is really useful for debating or explaining things.