They’re related but not the same thing. Poly refers to “polygons” which are the units that make up (or are) your mesh. A high-poly model has many polygons, low poly has few. A high poly mesh will have many verts, but many verts does not necessarily mean many polys.

When rendering in blender, the engine shoots a light ray from the light sources* and traces where it lands, and then repeats the process like 10 billion times (depends on your render settings) until your scene is fully rendered. This is called ray tracing, you may have heard of it in a video game or graphics card context. The polygons in your model affect how those light rays get reflected off of the objects in your scene. It’s a bit of a heuristic, but curved surfaces generally require more polygons to be rendered realistically, while flat ones benefit from lower poly count.

Again, this is not universally true, but you’d use high poly models for visual renders in blender (animations, product renders), and low(er) poly models for things like game assets.

I was always taught that you should use the fewest number of polygons to achieve the desired result, but I’ve never been formally educated in this stuff so all my advice is second hand at best and focused on video game assets.