What does a map with isobars show?

As water is cooled from 4 c to 0 c its density

How do you think the different landforms of northern and southern europe would affect ways of life in those two areas

This is a 2017 topographic map of puerto rico in the united states, and i'm sorry that people there have suffered such a disaster. i want to ask, what do the yellow dots and areas represent on this graph? and the blue area and the little blue circle, and the little blue sign that looks like a signal tower, what do they stand for?