Assuming you have some mail to read, type mail from the prompt. It tells you that Mail is waiting for a command. To read the waiting messages, use the type command which can be abbreviated to simply t. This is a demonstration of the mail utility. It can be used for all sorts of neat things, which will be listed in more detail somewhere else.
Mail has a lot of other commands, as well.
Having saved messages in the past, you can look through your mbox with the -f option of the Mail command. This will read in the contents of your mbox, and you can the continue as if you had just received the mail. You can then type, delete, reply to the messages, or anything else. When you again exit Mail, your mbox is updated to save any changes you have made to the messages.