Earth has a large metallic core. As the earth spins, that metallic core generates a huge magnetic field around the earth. It's like a force field protecting us from the sun's radiation. Solar storms and solar wind can hit that field with huge waves of highly-charged particles. When this happens we see the impact as aurora along the magnetic lines (aurora borealis in the north, aurora australis in the south). If the solar wind is strong enough, some of those charged particles can penetrate earth's magnetic field and play havoc with our electrical systems.