You can use this phrase when you want to discuss an imaginary situation. You start with "Let's say..." and then describe the imaginary situation. Then you ask questions about what would happen in that situation. For example:
Let's say another company offered you a 20% higher salary. Would you take their offer, or keep your current job?
Let's say you're travelling there and you get lost. Would you be able to speak the language fluently enough to find your way around?
"Let's say..." is especially useful when you're trying to persuade someone, like when you're selling things or having a debate.
Let's say I drop the price down to $150. Would you be interested then?