I don't discuss the rating system with a passenger unless they show genuine interest. Otherwise they just react negatively against what they see as begging for a 5*. If they are genuinely interested, you could explain the system like this:
After each trip, riders and drivers are given the opportunity to rate one another based on their trip experience. The rating system works to make sure that the most respectful riders and drivers are using Uber. Ratings are always reported as averages, and neither riders nor drivers will see the individual rating left for a particular trip.
When you rate your Uber trip, please remember that you're rating your driver only. You're not rating the Uber company, the Uber system, the Uber app, Uber's fares, Uber's "surge pricing", nor even the availability of Uber cars. These are all outside the control of the individual Uber driver. You are only rating that one driver on that particular trip.
If a driver's average rating falls below about 4.5, Uber will "deactivate" them, which means that the driver can no longer drive for Uber. If an Uber driver gets mostly 5 stars, their average will stay above 4.5 and they can continue driving. If they get mostly 4 stars, their average will drop below 4.5, they'll be deactivated, they're off the road, out of work, no more income.
If your Uber driver was courteous and got you to your destination safely and in good time, and their car was clean and well maintained, please consider rating them with 5 stars, so that they can continue driving.