Solar flares occur when magnetic fields on the Sun's surface get tangled and 'reconnect'. This is like rubber bands stretched taught being broken, and this causes plasma to be accelerated to very high speeds and temperatures. Yes, flares are a major problem for astronauts in space and can be lethal in terms of radiation doses. For humans on Earth it is not a problem unless you rely on satellites, which can sometimes get damaged.
Here are some useful web sites that discuss them in more detail:
What are they and how do scientists study them? NASA Marshall Space Flight Center Primer
Solar Flares and Space Weather SpaceWeather.com
Why study solar flares? NASA Goddard Solar Flare Page
Solar Proton Events 1976-2000 List
Copyright 1997 Dr. Sten Odenwald
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