When something sparkles, it shines and gives off little points of light. Sparkles are also the little points of light themselves.
When something sparkles, it is usually pretty and makes people happy.
Examples:
The stars sparkle in the night sky.
She wears a dress that sparkles in the light.
The water sparkles in the sun.
The diamond sparkles like a little star.
Her eyes sparkle with joy.
The sparkles on the Christmas tree make it look magical.