A coronal mass ejection (CME) is a significant release of plasma and accompanying magnetic field from the solar corona. They often follow solar flares and are normally present during a solar prominence eruption. The plasma is released into the solar wind, and can be observed in coronagraph imagery.
Are they rare events?
They do not happen regularly, but they do happen. The last such event was on October 14 (3 days ago):
"Yesterday, Oct. 14th, something exploded on the sun. Coronagraphs onboard the Solar and Heliospheric Observatory (SOHO) saw a cloud of debris (a CME) billowing over the edge of the solar disk. This CME will not hit Earth. It was launched from the farside of the sun, and the debris is therefore heading away from our planet. Sorry, auroraphiles! What exploded? Probably no t a sunspot. Sunspots are rare now because of an ongoing deep Solar Minimum. More likely, the source was an unstable filament of magnetism that whip-lashed in the sun's atmosphere, hurling surrounding gas into space. We can't say for sure, however, because the blast site was hidden behind the edge of the sun". (from Space Weather; see tools)
How devastating are these events?
Well, according to NASA they are nothing to worry about:
This seems like a PR exercise….not that I don’t trust a scientist with two earrings! It is true that a CME will not be a “killer” and humanity will survive. However, our electricity grid will probably not survive. Moreover, this assumes the protection of the earth magneto-sphere which can collapse with magnetic pole shift which is already happening. Our model of the sun is most probably wrong. An alternative model is based on electric-magnetic plasma rather than fusion. It is quite possible that the magnetic pole shift on earth is related to sun activity. A decreased or collapsing magnetic earth-shield also allows in more cosmic radiation which in turn effects cloud formation (therefore weather) and increases earthquake activity. All this is linked with our sun and our “electric-universe”.
The weird weather that we are experiencing now as well as the current Maunder minimum are probably the prelude to a cooling period which will become apparent next year (https://youtu.be/M_yqIj38UmY ). In other words there is a lot that we simply do not know. Satellites are protected but how confident can we really be? Electronic equipment is also “protected” (LOL).