Welcome,
RapidEars is a plugin for OpenEars which extends its functionality in one specific way, by doing recognition in realtime as the user is speaking instead of performing recognition after the user has completed their speech and paused for a certain period of time. To learn about how OpenEars uses a vocabulary that you define dynamically at runtime from an array of words or phrases, there is a good explanation in the documentation about the basics of offline speech recognition from the main OpenEars page and examples in the OpenEars tutorial and the sample app that ships with the OpenEars distribution.
The short answer is that all speech recognition uses a predefined list of words in a text file or memory structure of some kind. However, cloud-based speech recognition is able to use a much larger word list than recognition that is being performed on a handheld device because of the greater CPU and memory resources available, meaning that it can give the impression of recognizing “everything a user says”, while an offline SDK such as OpenEars needs to be used with a smaller chosen word set that applies to the specific task of the application rather than being generalized to any application.