I think Peter's right, that the ? isn't strictly necessary here, since the found string is already limited by the word boundaries and the character set to search on doesn't include punctuation or spaces. It won't hurt anything either.
Re: the + meaning one or more match, what this means is that it will find anything that begins with r and ends with t, no matter how many letters are in between. So all of the following would be found using that GREP:
rat
rot
root
robot
retirement
Jongware has an extremely helpful script called "What the GREP" (a free download) that will pull apart any GREP string and tell you exactly what it's doing:
http://www.jongware.com/idgrephelp.html
