What are northern lights?

An aurora, also known as polar lights, aurora polaris, northern lights, aurora borealis, or southern lights, aurora australis, is a natural light display in Earth’s sky, predominantly seen in high-latitude regions (around the Arctic and Antarctic).

Auroras display dynamic patterns of brilliant lights that appear as curtains, rays, spirals or dynamic flickers covering the entire sky.

Auroras are the result of disturbances in the magnetosphere caused by solar wind. These disturbances alter the trajectories of charged particles in the magnetospheric plasma. These particles, mainly electrons and protons, precipitate into the upper atmosphere (thermosphere/exosphere).
The resulting ionization and excitation of atmospheric constituents emit light of varying colour and complexity.
The form of the aurora, occurring within bands around both polar regions, is also dependent on the amount of acceleration imparted to the precipitating particles.

Most of the planets in the Solar System, some natural satellites, brown dwarfs, and even comets also host auroras.

* source Wikipedia

Where is the best place to see auroras

The best places to view aurora are high northern latitudes during the winter in Alaska, Canada, and Scandinavia.

To see aurora you need clear and dark sky. During very large auroral events, the aurora may be seen throughout the U.S. and Europe, but these events are rare.

Whats the best time to see auroras?

The best time to watch for aurora is the three or four hours around midnight, but aurora occurs throughout the night. Active auroral displays tend to be more diffuse and fragmented later in the night, which means that 9 p.m. to 3 a.m. is typically the time period with the highest probability of seeing spectacular auroral displays over interior Alaska in winter.  Since clear sky and darkness are both essential to see aurora, the best time is dictated by the weather and by the sunrise and sunset times. The moon is also very bright and can make it more difficult to view the aurora, so lunar cycles should be taken into account.

* source www.gi.alaska.edu