preg_match question

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In the php manual for preg_match:
http://us3.php.net/preg_match

What does the manual mean by this statement?
"$matches[1] will have the text that matched the first captured parenthesized subpattern"

More specifically, what is a "captured parenthesized subpattern"?

[NogDog]

http://www.php.net/manual/en/reference.pcre.pattern.syntax.php (scroll down to the "Subpatterns" heading)

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Thank you - that makes sense.