High-poly and low-poly refers to the polygon count of the 3D model - just like how we have low and high resolution images in 2D.
A high-poly model will be “heavier” and larger to store, like the file size of a PDF or Jpeg, and not suited for real-time uses such as gaming. Low-poly models are “lighter” and therefore easier to store, process, and interact with, but do not have such a high resolution. Each type has different advantages and disadvantages that make them a better choice for different use cases.