The northern hemisphere is most tilted toward the Sun in June, which is northern summer and why it is warm. The northern hemisphere is most tilted away from the Sun in December, which is northern hemisphere winter and why it is cold. The tilt changes the length of the day since the more than half of the hemisphere tilted toward the sun is in daylight, while less than half of the hemisphere tilted away is in daylight. The reason is clear if you imagine the tilt is 90 degrees instead of only 23. If it was 90 degrees then the north pole would point directly at the Sun in June and the entire southern hemisphere would be dark. At only 23 degrees, it is less than all or nothing. Try tilting a globe toward a light (or even toward to real Sun outdoors on a clear day) and see how it works.