AWeber's API uses a standard called OAuth to ensure that a request coming in to our API is made by a known integration and that the customer has given that integration permission to access their account.
An access token proves that the customer has given the integration permission to access their account because it is obtained by a developer after the customer successfully enters their AWeber login information during the OAuth process.
Some APIs require integrations to periodically refresh or replace their access tokens by giving them an expiration date, but AWeber does not have this requirement. Once you have obtained your access tokens they are yours to keep forever and will not expire.
Please be sure to keep your access tokens very safe and never share them, just like you would never share a password.