Currently, the rotation angle of the earth is 23.5 degree, if it rotates to 0 degree what will happen?
Biggest change would be no seasons would exist.
If the earth was rotating at 0 degrees, we wouldn't have noticeable seasons. When tilted towards the sun, it's "summer" and when tilted away it is "winter". If this tilt vanishes, then all positions in orbit around the sun will correspond to very similar temperature, conditions, etc...
This also has consequences for human life, because without seasons many necessary things (like farming) would not go so well.