Adding subscribers to your automation workflow
By default, imported contacts do not receive your automated email(s). If you go to the gear icon at the top right of your workflow, you will see the option "Should imported contacts receive these emails?".
If you set this to YES and click Save, then your imported subscribers will trigger the automation. Be aware that you need to have this set to YES before you import the subscribers or they will not receive your automated emails.
Automations are triggered when a subscriber is added to your list and your emails can be set to send automatically any hour or day after that event. If you add additional automated emails to your workflow after one or more emails have already been sent, the trigger is still the original time the subscriber was added to the list.
So if an existing subscriber was added 7 days ago and you add a new email set to send after 3 days, the existing subscriber won't receive it, but new subscribers will after 3 days. However, if you set the new email to send after 10 days, both the existing subscriber and new subscribers will receive it.