Also called the ordinate axis, it is the vertical axis along which the ordinate is measured and from which the abscissa is measured.
 
In a talent matrix or 9Box, the y-axis generally describes the employee's potential, which ascends from low to high, although the meaning of the y-axis can change based on organizational priorities.
We also talk about it in our blog:
How to make a 9-Box Talent Matrix (Nine Box)
How to make a 9-Box Talent Matrix (Nine Box)

Building a Nine Box Talent Matrix has never been easier. With Hrider's N-Box functionality, you can create 9-box matrices as well as 4-, 16-, and 25-quadrant matrices that correlate different variables such as performance and potential. In this tutorial you will learn what they are and how to easily create them step by step.