[michelpp]

This is great and a nice mathematical approach to the ideas of matrices. Another great resource is 3blue1brown's essence of linear algebra:

https://www.3blue1brown.com/essence-of-linear-algebra-page

Math is Fun also has a nice writeup that explain matrix multiplication from a real world example of a bakery making pies and tracking costs:

https://www.mathsisfun.com/algebra/matrix-multiplying.html