Here's what those phrases mean:
* "Let's agree" means to propose reaching a consensus or agreement.
* "It's agreed" means that a consensus or agreement has been reached.
For example:
* Let's agree to meet at 10 am tomorrow. (Proposal to reach an agreement)
* It's agreed. We will meet at 10 am tomorrow. (Confirmation that an agreement has been reached)
If you can provide more context about where you encountered the phrase "let agreed," I might be able to give you a more specific answer.