What does it mean to say two variables vary directly or inversely?

The statement, "two variables vary directly or inversely" show the relationship between the variables. 

(1) Vary directly.

The statement means that if one variable is increasing then, the value of the second variable is increasing as well.

(2) Vary inversely.

The statement means that if one variable is decreasing then, the value of the other one should be increasing or vice versa.