There are a number of definitions now that the term has come into more common use. What is arguably the most popular is its use to designate a girl who sleeps around a lot, and isn't particular about who she's with.
From Google:hood rat: A straight slut from the hood.. the kind or girl that has probably slept with you and four of your best homies... Usually lazy as hell ..they are known to party a lot.. easy to take advantage of.