What would happen to our seasons if Earth's axis was not tilted?
Seasons would not exist.
The sits at a tilt of 23.5 degrees to the solar plane. That side of the planet which is tilted towards the sun is enjoying its summer while that side which points away from the sun is in winter. At the spring and fall equinox, the sun is directly over the earths equator. Those lines referred to as the Tropic of Cancer and Tropic of Capricorn are the lines where the sun is directly overhead on either June 21 or December 21 respectively.