Client-side time is the time zone for each of your site visitors. It's based off the local time that their computer is set in.
For example if someone loads an article at 12:00pm (EST) in New York City, their client-side time is 12:00. At that same moment, someone in London would load that page at 5:00pm (BST) and their client-side time would be logged at 5:00pm.
UTC is a standardized way to denoting time across all time zones, used interchangeably with Greenwich Mean Time.