It's a fake grass mat but with a lot of colour changes throughout the scene. I've looked up grass tutorials but they all show how to make natural grass with just a uniform colour. The kind of colour palette I'm going for goes from light green (as seen here) to a light-blueish brown with several darker shades of green in between.
You could try something like this (bump mapped material option).
I just used a couple of noise textures mixed together to make a randomly patchy surface. I used a mapping node to stretch (scale) it a bit to make it longer like grass. I used the mix as the basis for a bump map. The reason I ran them through a MixRGB mixed with white first, is to soften the contrast, which would give to strong of a "bump effect" otherwise. Even after doing this, I still had to reduce the "distance" on the Bump Node. I also used a third noise texture which I used as a mix factor for the "main grass colors". You can control the mix by sliding the sliders on the ColorRamp.
I used an Ambient Occlusion node to give extra dark (shaded) patches, as well as to add a bit of a darker blue accent color. This can also be controlled by the ColorRamp. Usually, when using an Ambient Occlusion node, you should enable "Ambient Occlusion" in the render properties panel to get the full effect, but in this case, the "full effect" comes out too dark without further manipulation, so I left it unchecked. The node itself still provides values that can be used.
Lastly, I turned up the roughness and turned down the specular reflections to make it less "shiny". These may need to be adjusted to better suit your "model".
This is just a simple approximation of a grass shader, you can get much more detailed if necessary, this is just a starting example. I'll include the .blend file so you can play around with it without having to set it all up again yourself. Hopefully you can manipulate it successfully to your liking.