Tiny as they are they let out some of your feelings, and can be read together with other nonverbal cues. A great difference from many other nonverbal cues thou, is that you can't really control them with willpower - emotions slip out through your eyes, either you want to or not.
State and change because of physiological causes
The basic function of the pupils are to let in the right amount of light, like the blender in a camera: In a bright environment they become small, in a dark they become large. But they also reacts to some chemical substances and drugs: Alcohol and opiates makes them large while cocaine and LSD make them small. Aside from these physiological reactions their change may stem from emotional changes. First advice: When reading state and change in pupils you first need to filter out physiological causes.
State and change because of internal and external stimuli
It is not only what's seen - the brain also projects it's inner life through the pupils. Which makes for a second advice: Beside filtering out physiological reasons to the size of pupils you also have to consider the mental activities going on inside the head. Having no idea of which these are you need to be sure that the person you're reading has his/her thoughts here and now instead of somewhere else.
Large and small pupils
Pupils becomes large when the brain is reached by something interesting and small by something uninteresting or dislikable. Looking at an attractive person - yes, that is interesting. But also hearing something funny, trying to solve an exciting problem or remembering an amusing time. But some strong negative feeling also widen pupils, like anger and fear.
Pupils usually become small when the brain is reached by negative input or thoughts but also if it gets overloaded. Seeing something ugly, hearing something nasty, thinking of something bad, doing something unwanted and trying to solve a too difficult problem. Anxiety may be an underlying feeling here.
Summary
Filter out physiological causes as well as reactions from inner stimuli. After that you may get a glimpse of what the person you read feels about what's is said, shown and done at the present. Remember to include other nonverbal cues as well, to become more certain of your interpretation.
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