a. If two variables are positively associated, it means that they increase/ decrease together, i.e., when x increases y increases as well and when x decreases, y decreases.
For example,
As the temperature rises, people tend to consume more ice cream.
b. If two variables are negatively associated, then as one variable increases the other one decreases.
This means that when x increases y decreases and when x decreases, y increases.
For example,
The higher is the altitude, the less is the atmospheric pressure.