The PopImap client automates the fetching of your mail from remote POP or IMAP servers to your local machine. Once the mail is on your local machine, you can use any mail reader you like (rather than having to make do with one you don't really like that supports the needed client protocol).
The client really is little more than a scheduler for a unix mail-fetching command that you provide (it is not itself a POP or IMAP client; you need to get a mail-fetching client, on which see below). You configure the client by telling it what unix command will get your new mail from your remote server, and (optionally) what remote command will fetch an entire remote mailbox (including old messages), and replace a local mailbox with it.
One convenient feature is that the Net client knows about PopImap and vice versa, so that if both clients are used, PopImap doesn't try to check for mail when the Net line is down, and when the Net line goes up, Net schedules the periodic PopImap new mail checking to start right away (so that your new mail gets to you quickly).
In the preferences manager you will need to set preferences that indicate how frequently to look for new mail on the remote server, as well as the unix command that does the looking, and (optionally) the unix command that copies the entire remote mailbox over the local one (replacing it). The real work comes in setting up these unix commands, which may require finding some programs you don't yet have.
See also Biff's IMAP method, which doesn't require any external IMAP program.
Here I have a decided lack of expertise. I would very much appreciate pointers to stuff I don't know about.
All parameters are adjustable in the preferences manager.