RT audience score can be useful for certain movies that have a high self-selection audience. An example is the latest Guy Ritchie flick. People who go to see a Guy Ritchie film are generally fans who want to see a Guy Ritchie movie and all that entails. A high audience score tells you it hit the mark regardless of how the critics view the film.
I've always been a disciple of the Ebert school of review. it . I will watch anything from The Room to The Godfather and can enjoy it all with the right frame of mind. The critic and audience score combo will generally tell you what to expect as laid out by @Chuckie Finster, but I'd have different definitions.
High critic + High audience = well made, enjoyable on multiple levels. You might not like film if you don't like this.
High critic + Low audience = niche subject or plot, outside the realm of the vox populi. Possibly outstanding. Possibly horrendous.
Low critic + High audience = self-selecting audience who got what they were looking for and maker who couldn't care less about prestige. If you're one of the self-selecting, you'll love it. If not, you just don't get it, man.
Low critic + Low audience = Razzie nominee, possibly worth watching on an edible night.