The basics first.

The name of our Galaxy is "The Milky Way".  It consists of a bar-shaped core region surrounded by a disk of gas, dust, and stars forming four distinct arm structures spiralling outward in a logarithmic spiral shape. The galactic center harbors a compact object of very large mass, strongly suspected to be a supermassive black hole. Most galaxies are believed to have a supermassive black hole at their center.

Planet Earth is in orbit around a star named Sol (our sun). Our sun is one of billions of suns in The Milky Way Galaxy. Our sun orbits the center of the Milky Way Galaxy just as the Earth orbits Sol.

The actual three dimensional orbital path that our sun takes around the supermassive black hole located in the center of the Milky Way Galaxy would best be described as a "cork-screw motion within a sine wave". more>>> show-me  If we keep this discussion in just two dimensions it is easier to convey the general theory of why the Galactic Plane is so significant.

The two dimensions of interest are the sine wave portion of this orbital motion. The Milky Way is essentially very

flat as opposed to spherical like a ball. If you were to draw a line in the exact mid-point of The Milky Way Galaxy, that line would be called the "galactic plane". As you get closer to the galactic plane the density of stars, interstellar dust, and cosmic debris in general, increases. If our sine wave orbital path is relative to the galactic plane (as it is) then the diagram to the left illustrates this path.

As our sun approaches the galactic plane our entire solar system will be passing through this dense region of debris. "And here's the bad news. According to their calculations, the Solar System will be passing through the galactic plane in the near future, and should see an increased risk of impact. Our risk of impact could increase 10-fold." Full-Article 

It is believed the date of the crossing is December 21, 2012. This does not mean that that date is the only day of concern. We are increasing our odds of encountering this debris daily as we approach the center.

But wait, there's more...... Remember the object at the center of our galaxy is believed to be a supermassive black hole ? Well, scientists believe that such objects have some pretty special features. Features like massive X-Ray jets that are emitted from the poles of the supermassive black hole and an intense type of gravametric wave that emits from the equator of the supermassive black hole. So,....along with the increased debris field that exists along the galactic plane we can also expect to pass through this intense gravametric wave. The effects of this wave are expected to cause extreme solar flare activity, global warming on all planets in our solar system (as are being detected already), and the possibility of a magnetic pole shift (already being detected as well pole shift ).

Here are some more links on the subject.  more>>> Galactic Plane 1 (temporarily offline)

                                                                                   Galactic Plane 2

                                                                                   Galactic Plane 3