The imperative mood (often referred to simply as a command) is used to express demands, instructions or requests.
We usually use the second person (plural or singular) with an unspoken "you" for the subject.
When you're feeling bossy you can use the imperative.
Shut that door.
Turn that computer off now!
You can also tell people not to do something.
Don't eat that gummy bear!
This can all seem a bit rude, unless you are trying to save someone's life, but you can soften the command:-
Please, come in.
Shut the door, please.
The exeption to the use of the second person is when you want to include yourself in your suggestion, here we use "Let's ....".
Let's go to the pub.
If you can't see the difference between saying to someone "Go to the pub", and "Let's go to the pub," you might never get to go to the pub yourself.
Let's stop now, I'm getting a headache.