Grisaille is a fancy way of saying "Grey Painting." It's when an artist paints something using only shades of grey, black and white, and doesn't use any other colors.
It's like when you draw a picture using only a pencil, and you don't color it with any other colored pencils.
People use grisaille to practice their skills in painting, because it allows them to focus on getting the details right without having to worry about choosing colors.
It's also used in art to create a specific mood or effect, and can make a painting look old and classic, like an old black and white photo.
So whenever you see a painting that's only grey, black, and white, you can say it's a "grisaille."