What does a line of blue triangles mean on a weather map?
This is the standard symbol for a cold front.
Blue triangles represent a cold front advancing in the direction of the triangles, as cooler air displaces warmer air upwards. Red semicircles are a warm front, where warmer air pushes the cooler air away. A stationary front is represented by semicircles and triangles opposed to each other; the clashing air masses have temporarily battled to a standstill. An occluded front, semicircles and triangles directed the same way, occurs less often when a warm front and a cold front merge.
With any weather front, the clashing of different air temperatures leads to clouds and rain because moisture from the warm air over-saturated the cooler air. They are commonly associated with low pressure systems.
In temperate zones where weather systems generally move from west to east, the Coriolis force driven wind patterns around a low-pressure system drive warmer tropical air in front of the weather system; cooler polar air comes in from behind. Thus the passage of of a warm front tends to presage unsettled weather, whereas a cold front tends to be followed by clearing weather as the low pressure system moves away.