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(emacs-lisp-intro.info)fwd-para no fill prefix

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No fill prefix
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   It is simplest to look at the code for the case when there is no fill
prefix first.  This code consists of yet another inner `if' expression,
and reads as follows:

     (if (re-search-forward paragraph-start nil t)
         (goto-char (match-beginning 0))
       (goto-char (point-max)))

This expression actually does the work that most people think of as the
primary purpose of the `forward-paragraph' command: it causes a regular
expression search to occur that searches forward to the start of the
next paragraph and if it is found, moves point there; but if the start
of another paragraph if not found, it moves point to the end of the
accessible region of the buffer.

   The only unfamiliar part of this is the use of `match-beginning'.
This is another function that is new to us.  The `match-beginning'
function returns a number specifying the location of the start of the
text that was matched by the last regular expression search.

   The `match-beginning' function is used here because of a
characteristic of a forward search: a successful forward search,
regardless of whether it is a plain search or a regular expression
search, will move point to the end of the text that is found.  In this
case, a successful search will move point to the end of the pattern for
`paragraph-start', which will be the beginning of the next paragraph
rather than the end of the current one.

   However, we want to put point at the end of the current paragraph,
not at the beginning of the next one.  The two positions may be
different, because there may be several blank lines between paragraphs.

   When given an argument of 0, `match-beginning' returns the position
that is the start of the text that the most recent regular expression
search matched.  In this case, the most recent regular expression
search is the one looking for `paragraph-start', so `match-beginning'
returns the beginning position of the pattern, rather than the end of
the pattern.  The beginning position is the end of the paragraph.

   (Incidentally, when passed a positive number as an argument, the
`match-beginning' function will place point at that parenthesized
expression in the last regular expression.  It is a useful function.)