The incoming mail server is a computer on which your ISP holds email that's been sent to you. It stays there until you collect it.
First, set the account type. Your ISP will tell you if it's POP or IMAP - usually POP.
Next, a description for this email account - it can be anything you like.
Now the name of the incoming server as provided by your ISP. It will usually have a name that starts with 'mail' or perhaps 'pop' and goes on with your ISP's name, each part separated with a dot. Shown here is mail.anyisp.com
Your user name may be your real name as all one word as here or it may be your email address. When we say 'may be' it doesn't mean you have a free choice - you must enter exactly what your ISP said your user name is.
Finally, your password which will be filled already from the one you entered in the previous step.
When you press 'Continue', Mail will try to connect to check that all is well. If it can't, double check that you have entered the information exactly as your ISP gave it to you.