What do you mean there is a linear relationship between them? Popular explanation

When there is a functional relationship between two variables, there is a linear relationship between them.

Numerically speaking, if one variable is 1, 2, 3, 4, 5 and the other variable is 2.4.6.8.10, there is a linear relationship between the two groups of variables;

According to the equation, if nX+m=Y, (n is a constant (n is not equal to 0) and m is an arbitrary number), there is a linear relationship between variable X and variable Y;

From the image, if these two variables are taken as the abscissa and ordinate of a point respectively, and the image is a straight line on the plane, then the relationship between these two variables is linear.