"Theory sessions" refer to classes, lectures, or discussions where theoretical knowledge or concepts are taught and explained without practical implementation. Full definition
The very first beginner lesson begin with a 15 minute theory session on the beach followed by two practical classes in the water. (costaricasurfing.org)
There will be a short theory session in which you will learn the important basic rules for scuba diving (dealing with water pressure, breathing on scuba, etc.). (edenrockdive.com)
There will be a classroom theory session on each of these days along with hands on equipment configuration and your dives. (seaventuresdive.com)