A CME is a massive cloud burst of electromagnetic radiation consiting of charged particles such as electrons and protons, along with other elements from the sun's surface. The Earth's magnetic field shields us from these charged particles, but some of them get trapped by the magnetic field, and are then drawn in towards the Earth's poles along magnetic field lines.
As the charged particles from the sun collide with atoms in the Earth's atmosphere, mainly oxygen and nitrogen, the interaction between them produces brilliant colors of light. More information on viewing can be found here, as well as updates on the progess of the CME moving towards the Earth. Clouds will be more abundant Thursday night into Friday morning, so we'll have a higher chance of seeing the northern lights tonight with less cloud cover.