Those plain lines that curve across a weather map are called isobars. (iso = equal, bar = pressure). They join together places with the same air pressure, and are measured in hectoPascals. HectoPascals measure how much the air is pressing down, or how heavy it is.

If the numbers are low, it is an area of low pressure.

If the numbers are high, it is an area of high pressure.

Think of the lines like contour lines on a map, showing how high the land is.