Nested toys are a simple introduction to early maths.
Babies learn that some objects fit into holes but not others. They are developing an understanding of big and small. Next comes the need to put three objects in order: small, medium and large; big, bigger, biggest. Think the Three Bears' porridge bowls or chairs.
Nested toys are brilliant because they teach seriation, the idea that you can organise objects according to characteristics such as height or length. Rather than small, medium and large, you can learn to order ten or even twelve items (if you're using a rainbow). They are what's known as a 'didactic' material. This means that they teach you without the need for an adult. You learn the concept simply by playing with the toy.
Once you know that stacking toys must be completed in a set order, it's much easier to understand that numbers work in the same way. 1, 2, 3, 4, 5... The order never changes.