They have the nicest sounding little happy "chirp" don't they? Usually it means they are happy, excited, wanting to play or socialize - it's just a pleasant, passive communication sound you'll hear quite a bit in happy societies of cavies.
Males and females may include these chirps in their bonding or mating ritual, moms and dads use it to communicate positives and negatives to their offspring.
Sometimes though, it could be a distress chirp, meaning they are in trouble, not feeling well or are frightened. The longer the owner has their piggy, the more language the owner learns.
There's a behavior unique to cavies called "popcorning". When some owners see this it can be of concern, very scary looking, like a spasm or seizure. What it really portrays is a very content and happy piggy