Chiaroscuro is a big word that means "light and dark." Remember when you turned off the light in your room and it got really dark? Chiaroscuro is when an artist uses different shades of light and dark to make their art look more realistic or dramatic.
Think about a picture of a person's face. The artist might use darker shades of paint to show where the shadows are, like under the person's nose, under their chin, or around their eyes. Then they might use lighter shades of paint to show where the light is hitting their face, like on their forehead or cheeks.
By using different shades of light and dark, the artist can make their picture look like it has depth and shape, and can make it look like the light is actually hitting the person in the picture. This is what we call chiaroscuro!