The complete form is let us.

Let's means "let us, let's (come)" and is used to make suggestions or invite someone to do something together, including the listener. Its full form, Let us, means "let us" and is used to request or ask, excluding the listener. For example: Let's go watch TV (Mom, let's watch TV together). If let us is used, it should be translated as: "Mom, let us watch TV."