Basic shapes are the foundation for so many things that little ones learn. For example, a precursor to writing is drawing shapes. Children have to master moving the pencil deliberately to create something before they can fine tune that to make letters. Shapes are a precursor to advanced drawing. If children can see how complex figures are made up of simple shapes, then they can draw them by combining basic shapes into something complex. Recognizing shapes is a precursor to literacy. Letters are just more complicated shapes.
Here's a fun song to introduce shapes and their definitions. You can use objects with your child as you sing the song to show a real-life example of the shape.