What's z in a surface trace?

Plotly R, MATLAB, Julia, Net Plotly R
1

According to Single-page reference in R

The data the describes the coordinates of the surface is set in z. Data in z should be a 2D list. Coordinates in x and y can either be 1D lists or {2D arrays} (e.g. to graph parametric surfaces). If not provided in x and y, the x and y coordinates are assumed to be linear starting at 0 with a unit step. The color scale corresponds to the z values by default. For custom color scales, use surfacecolor which should be a 2D list, where its bounds can be controlled using cmin and cmax.

What is z exactly? I expected it to be z values, so I would expect it to be 1D, but apparently it’s expecting a 2D array? What do I plug in there?