Temperature affects firearm ballistics in many ways. It directly affects the speed of sound. It can affect the burn rate or pressure of the burning charge in ammunition. It also affects the trajectory of the bullet in flight. High temperature tends to decrease air density. With air that is less dense the bullet as it travels encounters less resistance and is able to conserve its velocity longer.
If you do not check the 'Correct for Atmosphere' checkbox then the International Standard Atmosphere temperature of 59° F will be used.
The Weather Underground is a good website to use to find out the altitude, pressure, temperature, and humidity for where you live. Plug those numbers into the ballistic trajectory calculator in order to have more accurate results.