A catchall is an account that receives ALL email that is sent to your domain that isn't addressed to a live account. For example, if sales@YourWebSite.com is your catchall account, and someone sends an email to "abc123@YourWebSite.com" or "xxxx@YourWebSite.com" (these being invalid addresses), their email message would go to your catchall account (sales@YourWebSite.com). Normally—if you the catchall is set to "bounce"—all messages that are not addressed to a real email account would bounce.
We don't recommend setting up a catchall, as it will mostly catch spam. i.e. Setting up the catchall to go to a real email account is not advised. Either set it to "Catchall - Bounced" or "Catchall - Deleted."