Best explained here
https://notes.nicolevanderhoeven.com/Observability
Observability is the measure of how much can be inferred about the inner workings of an application based on its ouputs. [1] As a practice and a computing science, observability involves setting up applications to produce outputs that can be studied, analyzing the outputs, and improving the application based on insights learned from the outputs. True observability involves gathering more information about a system than you need, so that you can answer not just expected questions but also unexpected ones. [2]