Fresco is a kind of art that is made by painting on wet plaster. Imagine that you have a big cake and you want to decorate it with some frosting. You have to put the frosting on the cake while it's still wet, otherwise it won't stick. The same thing happens with fresco art. The artist paints on the wet plaster and the paint sticks to it. When the plaster dries, the painting becomes part of the wall or ceiling.
Fresco art has been around for a very long time. People used to make frescoes on the walls of their houses or public buildings to tell stories or to show beautiful pictures. Some of the most famous frescoes in the world were painted by a man named Michelangelo. He painted amazing frescoes on the ceiling of the Sistine Chapel in Italy. If you ever go there, you can look up and see his beautiful art!
The cool thing about fresco art is that it lasts a really long time! Because the paint becomes part of the plaster, it's hard to chip or peel off. That means that people can still see frescoes that were painted hundreds of years ago. So, fresco art is like a yummy cake that lasts a really, really long time!