If The Earth was standing up right then every day would be twelve hours long and every month would be the same temperature. There would be no winter and no summer.
But the Earth is not standing straight up! It is tilted slightly, about 23 ½ degrees from true north and south. It's this tilt that causes the seasons, and not the distance from the Earth to the Sun. So as the Earth travels around the Sun, in June the Northern part of the Earth is leaning closer to the Sun causing long days and warmer weather.
Likewise during December the Southern part is tilted towards the Sun giving that part of the earth longer days and warmer weather. At the North and South poles this tilt causes the Sun to stay in the sky 24 hours a day during summer. It doesn't even set in the middle of the night!
The opposite happens in the winter. The Sun doesn't rise at the poles meaning that they're in constant darkness. We call these days when the earth is at its most extreme, the Solstice. For the Northern hemisphere the Summer solstice occurs on June 21st and the winter solstice on December 21st. So between these two dates there must be a period when the earth is equal right across the whole planet. These days are called the Equinox and occur on March 21st and September 21st each year.
If you draw an imaginary line running around the Earth at the center we call it the equator. This is half way between the north and south poles. On both equinoxes the Sun will be directly overhead here.
At 23 ½ degrees north we have a line called the Tropic…