What does the space on the resultant graph mean?

Hello everyone. I have done 2 codes to demonstrate the 2D heat diffusion through an aluminium plate using the Explicit Finite Difference Method. However, on the first code, when I change the grid spacing and time step to see how the 2 influence the code, I obtained this:

On the first one I chose a smaller grid spacing and time step than on the second imagine. However, what does that space means?