"tilted on its axis" is a meaningless phrase, and I wish everybody
would just stop saying it.
The axis of anything that's spinning is the imaginary line around which
it's spinning. The Earth's axis is the imaginary line through the north
and south poles, and that's the line that the Earth spins around.
The next question is: Which way does the Earth's axis point ?
Does it point straight at the sun ? Obviously not.
Does it point "straight up and down", perpendicular to the direction
to the sun ? No, not that way either. The axis is not 90 degrees from
the direction to the sun, it's only 67.5 degrees from that direction. So
the axis is not "straight up and down", and THAT's why we say it's tilted.
