Its finding a string between to other strings. :-)
It finds a start tag position, finds the end tag position, gets the stuff inbetween, and keeps doing it until either there arn't any more matches, or $number ( the 4th param ) is reached. For instance, to find the bolded words in this HTML:
<?php
$html = " this is my text and this <b> is in bold </b> but this isn't ";
$array = StrBetweenStr ($html, "<b>", "</b>");
print_r ($array);
// prints " is in bold "
?>