The "catchall" email account is where all incoming email sent to your domain which isn't addressed to an existing email account is directed.
An incoming email addressed to firstname.lastname@example.org arrives at our email servers. You have not created an email account by the name of "support" therefore the email is routed to the inbox of your catchall account.
By default the catchall account will initially accrue email in the default account inbox, however this can be changed to any email address you prefer, or it can be either deleted ("discard"), or returned to sender aka 'bounced' ("Discard with error to sender"). For more details on these options see the "Options for handling undeliverable (catchall) email" FAQ under this same topic section.
For further information:
Please refer to the cPanel documentation for Default Accounts, here.