When subscribers are added to your list, by default they will be sent the legacy follow up messages in your Legacy Follow Up Series. Legacy Follow Up Series are completely separate from Campaigns, so if you did not wish to use one or the other, you're not required to do so.
If you would like subscribers to receive your campaign in addition to your legacy follow up messages, you will need to trigger your campaign based on either the "On Subscribe" or "Tag Applied" trigger. Campaigns allows you to choose which subscribers will receive the campaign whereas Legacy Follow Up Series messages are sent to all active subscribers added to your list.
Legacy Follow Up Series
A Legacy Follow Up Series consists of messages that are sent in sequential order based on the time intervals you have set. With legacy follow ups, the first legacy follow up message is immediately sent to subscribers after they're actively subscribed to the list.
Campaigns is AWeber's email automation platform, which allows you add subscribers into the campaign based on your selected trigger. Once in the campaign, subscribers can receive messages, adhere to a schedule, and have tags applied.