The concept of an AI Robot and a AI Robot Pro is the legacy licensing method used by UiPath AI Center^TM.

An AI Robot is the runtime for serving ML Skills (machine learning models available for robots to make prediction requests) and run ML Training jobs (training a new model version on new data to).
One AI Robot can serve two ML Skills or run one ML training job. Any user connected to the cloud Orchestrator instance can access ML Skills.

An AI Robot Pro is the same concept as an AI Robot with the addition that the runtime can use a GPU (for both serving a model or training a model). A GPU is just a different processing unit that speeds up some types of computations. The efficiency gain by using a GPU runtime is heavily dependent on the code being run.