Autoresponder refers to a type of email that is sent out as an automatic response. This type of email may be triggered in response to an action such as downloading something from a website or signing up to be on an email list. It may also be pre-scheduled to send out at a specific time.
The automatic email can be as simple as a thank you for downloading (or whatever action the person has taken) or it may include additional information. For example, "Thank you for signing up to use our services. This email confirms that your account has been successfully created. Your account number is 70132."