When two variables are perfectly linearly related, the points of a scatterplot fall on a straight line as shown below.

If you know the score of a subject on one variable then you can determine the score on the other variable exactly. With behavioral data, there is almost never a perfect linear relationship between two variables. The more the points tend to fall along a straight line the stronger the linear relationship. The figure below shows two variables (X and Y) that have a strong but not a perfect linear relationship.