Let's start at the Preface:
Meaning of UAI
This thesis concerns the optimal behaviour of agents in unknown computable environments, also known as universal artificial intelligence [UAI]. These theoretical agents are able to learn to perform optimally in many types of environments... Moreover, these agents can be proven to upper bound the performance of general purpose computable agents.Okay, the first paragraph is heavy, and summarizes the whole point of UAI. Let me unpack it for you.
