AWeber is an email marketing service. It sends newsletters, runs autoresponder sequences, and hosts signup forms and landing pages that add a person to a list. The account holder writes the message and chooses the list. AWeber handles the sending.
A form on a website can subscribe someone only after that person submits it. Templates, automations, and the size of the list are what the plans differ on, and those limits are on AWeber’s pricing page. Deliverability still depends on the content of the mail and on whether the list was collected with permission.
This is a tool for a person or a business that already has something to send. It is not a source of addresses. Importing a list the recipients did not ask for is outside what the service is for, and AWeber’s own rules cover that.