The number rods are genious in their connection to the child's sensory work and the simplicity with which they introduce abstract mathematical concepts to the child. The presentation you will see in the video is a later presentation, when the child already understands bound quantities and knows the individual symbols. This presentation connects the two knowledge. The child will become aware of the substitution of a symbol for a quantity. He gains experience with quantity. He visually sees how quantity increases by a regular length - that is, by one. Through colour highlighting, the child sees that the larger number contains all the smaller quantities. The child gains experience of counting by ones to ten. The child learns the basics of the decimal system. By comparing, the child can later gain experience with mathematical operations as well. Genius!
Enjoy Genius Montessori Materials, Episode 9: Number Rods.