Sterile Cockpit is something that professional cockpits do to cut down on non-operational talk. The Sterile light for the airline I worked for , as a flight attendant, was over the cockpit door. We could not call them UNLESS it was an serious emergency, because they were in their busiest phase of flight.
What is Sterile Cockpit? Well for the airlines it is the time from push-back to 10,000ft (and on the descent also) where non-operational talk is discouraged. To state the obvious take off and landing are the busiest time and it is important to keep focus on the task at hand.
How does that relate to General Aviation? It is exactly the same except , we may not go to 10,000ft :) . Some examples of how you can be easily distracted when flying
1- as you know when you are flying with an instructor, you often have become friends and it is natural to want to catch up.
2-if it is a BFR you may be learning about each other and how the flight will progress.
3-if you are flying with family or friends, they have so many questions and it is fun to share the flight with them.
My suggestion is that as pilot in command that you brief your passengers (yes! your instructor too) on phases of flight where you will not be able to chit chat i.e. landing, take off or whenever you feel you need your attention on flying. Obviously your instructor is instructing you and that is necessary but catching up on the Super Bowl and the like can be done before the flight. Adding this to your passenger briefing will ease your passengers and show them you are flying professionally and safe even on a fun trip to have lunch.