Flat design is a style of design where you don't use any decorations or special effects like shadows or textures to make things look more realistic. Instead, you use bright colors and simple shapes, with the goal of making things clear and easy to use. It's like the difference between a drawing of a house made with a crayon and one made with a computer - the crayon version is flat, while the computer version is 3D. Flat design can make websites, apps, and other digital content look modern, clean and easy to use.