This is because the Earth is spinning on its axis, as well as orbiting the sun. It completes a full rotation on its axis once every 24 hours (ie once a day). This means that for some of each 24 hours, wherever you are in the world, you will have sunlight, and other hours it will be dark. We then have timezones so that wherever you are in the world, everyone’s ‘daylight hours’ are consistent.
As Alexandra says, it because the Earth is a spinning globe. It has imaginary lines which run from the North to South poles which we use to determine our time zones (approximately). The line which all time zones are based on runs through Greenwich in London. Time zones to the east of Greenwich (like Australia) are ahead of Greenwich time (+1 hr), while time zones on the west side (like America) are behind (-1 hr). So if it was 12 pm in Greenwich, it would be 8 pm in Australia and 7 am in Florida.