If you have taken my photography workshop, we talk about catchlights there too. Catchlights are a great indicator of how your image will turn out, and how the light is in your image.
So, what are catchlights?
Catchlights are the reflections of your light source in your subject's eyes.
This can be the reflection of a camera flash, the reflection of an overhead or lamp light indoors, or the reflection of the sky outside, to name a few.
Why do catchlights matter?
Well, for one, they make your photo a whole lot prettier. When there are no catchlights in the eyes, they appear dull and flat. Kind of lifeless in a way. As soon as there is light reflected in them, causing those catchlights to appear, it's an instantaneous change, and the eyes seem to sparkle!
As you take photos in your daily life, start to pay attention to your subject's eyes. If you don't see those catchlights, try spinning them slowly in a circle, watching to see if one direction causes more catchlights than another. Usually, there is one direction that will have better light, causing better catchlights than others.
Test it out. See if it makes a difference in your images.