What causes high and low pressure?
We could spend pages discussing this, but let me give you the simplified but still correct version. High pressure is an area of sinking air. It's caused by air converging, or coming together higher up in the atmosphere, which causes the sinking, then spreading out near the surface. It's similar to placing more weights on a scale, which is why the air pressure rises. Incidentally, this sinking motion leads to warming and drying within the air mass, which is why high pressure systems often (but not always), lead to clearing.
Low pressure marks an area of rising air. It's caused by air converging at the surface, which causes the rising, then spreading out higher up in the atmosphere-the exact opposite of what's going on within a high. It's similar to taking weights off a scale, which is why the air pressure decreases. This rising motion leads to cooling and condensation of moisture within the air mass, which is why low pressure system often (but not always), lead to clouds and precipitation.