The violent explosion of a sun flare can trigger dangerous eruption called coronal mass ejection or CME. In CME, energy from solar flare cause highly charge radioactive plasma to spread out from the solar atmosphere into the cosmos. They can travel as fast as 800/900 miles per sec. It expels a massive amount of material into the space.
Sometimes these super heated radioactive plasma travel towards earth and hit our atmosphere. This event is also known as solar storm. These storms can induce electrical current in outer atmosphere that can damage satellite and power grids.
Solar storm consists of charged particle so they are initially diverted around the planet by Earth's magnetic field. If there were no magnetic field it would have hit atmosphere directly and as a result our atmosphere would have thrown out into the space. One reason Mars does not have any atmosphere because it doesn't have strong internal magnetic field. That's why gradually over millions of years Mar's atmosphere was gradually knocked off into the space.
But Earth's magnetic field is not perfect to protect us from solar storm. Some particle can still penetrate it which charge up the upper atmosphere. Solar storm can even bend and break Earth's magnetic field lines on the far side of the Earth. This allows charged particle to zip back down the field lines towards the north and south poles. Extremely powerful storm distort magnetic field even further that induce electrical current. When this happen technology like long distant power lines can become overloaded and transformer can be damaged.
Sun spot act as the trigger of the most severe solar system. We can see CME ejecting from sun all the time. But the CME happening behind the sun is not our concern. CME erupting from the center of the sun is dangerous for Earth. As the sun spot rotates around and it faces directly towards earth that is when we really have to worry about a solar storm. Then the big storm can travel directly to earth and full force of storm slams directly to Earth.