It's just what it sounds like it is. This is the server where your incoming mail is stored until you pick it up. The address will usually be in the format "mail.adomain.com". If your ISP and your e-mail address domains are the same, your outgoing mail server will usually be in the format "smtp.adomain.com". If your domain (assuming you own one) is hosted elsewhere, the outgoing mail server generally has to be your ISP's outgoing mail server, e.g., "smtp.myisp.com" and your incoming mail server will be "mail.mydomain.com."