The default email address, also known as a 'catch all', receives email sent to accounts that don't exist at your domain.
The default access is set to 'fail: no such address here' in cPanel by default. You can set the default address to any address you like, but be aware that this address will probably receive large amounts of spam and be susceptible to exceeding the email or disk space quota.
We recommend that all clients leave the default email address disabled.