Pick a date and time in one place and read it in the others — "3 pm in Chicago is what in London, Tokyo, and Sydney?" answered in one table.
Conversions use the browser’s built-in IANA time zone database, so daylight saving changes are handled correctly for the date chosen — including the weeks when the US and Europe are out of sync. Times are shown in 24-hour format with the zone’s UTC offset for that date.
Why does the London difference change during the year? The US and UK change clocks on different dates, so for a few weeks in March and October the gap is an hour shorter than usual. The converter uses the actual date, so it shows the right gap.
What does UTC mean here? Coordinated Universal Time — the reference all zones are offset from. It never observes daylight saving.