The easiest way to visualize how this works is to think of it like real postal mail. When you send a letter, you send it in an envelope, that envelope contains your message. The return address written on the postal mail is the same as the MAIL FROM: return address and the RCPT TO: will give the recipient address. This is all part of the envelope information.
The message content (inside the postal mail envelope) is like the body of your message. In the email sense, it can also have the header field information such as Subject, Date, the To, and From.
The MAIL FROM: command specifies the address for return purposes, for example, if there are issues with mail delivery.
The From: header field will tell you who is the author of the message.