Answer:
Depends on which lines you're talking about. If you're talking about circles that cover the map (often in black or gray) then those are isobars which are lines of equal pressure (ie. the barometer reads the same everywhere along a line)....If you're talking about a blue line with triangles, that means a cold front with cooler/drier air behind it. A red line with round barbs means a warm front with warmer air behind it. There are many others, but these are the most common seen on television weather forecasts.