Grool is a word created from the fusion of 'girl' and 'drool'. It refers to the material created by aroused women that shows that they are genuinely aroused and looking to get pumped full of rock hard cock. The premise of this place is really simple: people post amateur and professional pictures alike of ladies that are exhibiting signs of grolliness and really just want to get fucked as hard as possible with their super moist pussies begging for all of the action.
/r/Grool has 60,000 subscribers and new posts are made about once every two hours. I'd say that around 80% of the material is amateur and user-submitted, so these are images produced by users of Reddit that want to share the creamy goodness with the world. While the subreddit has quite a few rules, most of them are just to ensure the quality of the posts and to maintain high quality grool submissions. Anyway, my only advice is to head on over as soon as possible to get your hands on the yucky stuff - you won't be disappointed!