Can I turn a procedural displacement into geometry? I was following this tutorial to create procedural rocks: https://www.youtube.com/watch?v=UFsVzDXYOKw

It gives some really nice results (that I admittedly don't fully understand), the only problem is that those results only exist when rendered.  The only thing physically there is the sphere that the many, many nodes get applied to. My ultimate goal is to export the rocks to a game engine, so they need to become actual geometry as at least a first step.  But it's not clear to me how I'd go about this.  Any thoughts?
Yes, you can.

But you need to use different modifiers for this.

First, remove subsurf and use multires+displacement:



You need to use multires for simplicity in creating normal map for game engine. With normal map you can create a detailed object without complex geometry, that important for game engines.



For creating normal map, unwrap your object (U), crate material and put image for baking in it, and in render settings select normal type and press bake.



After baking, save resulted image and setup it as normal map. Now you can remove multires, apply displace (for basic shape) and export your model in game engine.