How to Find an IMAP Server
The IMAP email protocol is a set of rules and standards for computers to talk to each other: in this case, specifically to upload and download email messages to and from a remote server. If you're using an email client like Microsoft Outlook or Mozilla Thunderbird, you can set it to connect to your email provider's server via IMAP, and send and receive emails directly rather than going through the Web. To do so, you need the address of your provider's IMAP server, and any additional information about their IMAP support.
Make sure your email provider supports IMAP. Providers who only support POP3 are becoming increasingly rare, but you should still go to your provider's website and look for a general description of the service--it should mention what protocols it supports.
Enable IMAP support, if necessary. Gmail, for example, requires an "Enable IMAP" option to be set in its online Preferences screen before it will allow clients to connect to its servers over an IMAP protocol. Check your provider's documentation for information on this.
Find the IMAP server address. This is often something like "imap.provider.com," where "provider" is the name of your provider--but, again, you should find the address on the website. If there's no sign of it, contact someone directly and ask for information about their IMAP server.
Enter the IMAP server and login information into the Settings screen of whatever email client you're using.