A campaign is a program that runs a specific message using specific rules and timing.
Each campaign will run only once a day based on the specific timing rules you've selected.
When you activate a campaign, it will target all future orders. You can target past orders by selecting target past orders when you select "save and send" a pop up will appear.
You can run as many campaigns as you want at once but be sure to follow the Amazon TOS.
There are two types of campaigns the Custom Email Campaign and the Review Request campaign.
For information on which type of campaign you should use, refer to this article.