High levels of solar activity can heat the Earth's upper atmosphere, causing it to expand and producing more drag on satellites in low Earth orbit. The flares also accelerate charged particles which produce aurorae, or northern lights, when they get trapped by the Earth's magnetic field. Both the radiation and the charged particles are dangerous to astronauts in Earth orbit. Exposure to solar flares can increase the risk of an astronaut to getting cancer or to having children with birth defects. Therefore, spacecraft and spacesuits are designed with shielding to protect the astronauts.
Solar flares have little, if any, effect on the Earth's weather.