A Geomagnetic storm is a blast of solar wind ejected from the sun. They are also known as "coronal mass ejections" or CME's for short. This current blast of plasma left our star on April 10th and we are now passing through the solar wind stream. Scientists are calling this CME a G1-class. This is the weakest of all geomagnetic storms.
The incoming solar wind stream is pouring out of a hole in the sun's atmosphere--a "coronal hole"--shown in the image below from NASA's Solar Dynamics Observatory:
These "Coronal holes" rip open the sun's surface, permitting solar winds to blast into space. The image above also shows the flow of the solar wind.
What does all this mean?
CME's can be harmful, but the earth has its own protective shield in the form of a magnetic field. This field deflects those solar winds around the planet but not the whole blast. Some, is directed to our poles and get turned into Auroras, or polar lights. Astronomers say the northern latitudes may get that light show tonight and possibly stick around through Wednesday as well.
Outside of some Northern Lights, this CME, will be harmless.