Solid line + values = mean sea level pressure isobars each 5 hPa (hectopascal).
Thick solid line with wedges = cold front moving in the direction of pointing.
Thick solid line with semicircles = warm front moving in the direction of pointing.
Thick solid line with wedges and semicircles = occluded front or occlusion moving in the direction of pointing.
Thick solid line = trough and convergence line.
H = high pressure
T = low pressure

Useful for: fronts usually associated with clouds sometimes accompanied by precipitation. Behind warm front = warm air advection in upper air (usually also at surface level). Behind cold front = advection of cold air in upper levels (usually also at surface level).
Useful for: prognoses for wind speed (tight isobars =  rather windy), direction = clock wise around high pressure and anti clock wise around low pressure, and general type of weather (rather poor in a low, and fine in a high).