Agar plates are culture medium plates. A gelatinous substance obtained from algae (Agar Agar) gives the nutrient solution stability and a smooth surface, just like pudding or jelly. The bacteria or fungi grow on the surface and are nourished by diffusion from below.
Explanation for children:
Agar plates are most commonly found in the laboratory. These are plates with a solid culture medium on which the microorganisms are placed. Then the microorganisms grow on the surface and after a few days you can see nice colors or spots of microorganisms. You can see them because each dot consists of thousands or millions of bacteria.