An isobar is a line drawn on a meteorological map joining points which are, at a given moment in time, experiencing the same air pressure. Isobars connects points of equal pressure at the Earth's surface or at a constant height above it. Numbers written on isobars give the atmospheric pressure measured in millibars.
The closer the isobars, the stronger the wind will be, the further away they are, the lighter the wind will be. Generally, isobars are tightly wrapped around regions of low pressure (so it is windy there as well as wet and cloudy), but well apart around regions of high pressure (so winds there are light breezes or calm).