The basics
In winter, the polar vortex forms over the polar region. The polar vortex is a thermal low pressure area. In contrast to the dynamic lows that travel in our latitudes, the polar vortex is formed purely thermally, i.e. by the temperature. This drops rapidly at the North Pole in winter due to the lack of solar radiation. Cold air is denser and therefore heavier than warm air and sinks downwards. In winter, a lot of air flows from higher layers towards the ground at the North Pole. This causes the air pressure on the ground to rise, while it sinks at altitude. This creates an altitude low, also known as a polar vortex.
The polar vortex, like all other lows, rotates anti-clockwise in the northern hemisphere. At the transition between the cold, polar air masses in the north and the milder air masses in the mid-latitudes, the strong pressure differences create a strong high-altitude current, the so-called jet stream.
Over the Arctic, the polar vortex often has a structure with two or more centers that waft around in a loose network due to the uneven land/water distribution. There is often one center over Baffin Island and one over northeast Siberia. The closer the pressure centers are to each other, or the rounder the polar vortex is, the straighter or more zonal the west-east flow is. If the centers are further apart or irregularly shifted, pronounced waves form in the jet stream. Instead of running directly from west to east, the upper-level current meanders and loops - this is also referred to as a meridional flow.
The latter situation often involves blocking layers, in which a wedge or trough settles somewhere more or less stationary and does not move away so quickly because the driving west-east current is missing.