Replying (R) and forwarding (F) are your two options for following up on the message you are reading. You would use reply if you want to get email back to the author of the message and/or the other people who have already seen it. You use forward if you want somebody new to see the message.
In the normal case, the only thing that you must supply when forwarding a message is the name/email address of the new recipient. Pine will include the text of the forwarded message. Pine will also include any attachments to the message if you have requested them. There is space above the forwarded text for you to include any comments.
When replying, you usually have to answer some questions. If the
message is to multiple people and/or specified with a
Reply-to: header, then you will have to decide who should
get the reply. You also need to decide whether or not to include
the previous message in your reply. Some of this is configurable.
Specifically, see the include-header-in-reply and
include-text-in-reply configuration features.
Both the reply and forward commands react to the full header mode toggle. If the full header mode is on, then all the header and delivery lines are included with the text of the message in your reply/forward.