What is a straight line graph?

It is the graphical representation of a linear relationship between two variables.

In its most general form, the relationship is of the form

a1x1 + a2x2 + ... + anxn + c = 0 where x1, x2 etc are variables, and the as are constants. This equation would be a straight line in n-dimensional space.

In 2-dimensional space, and renaming the variables, you get

ax + by + c = 0 where x and y are the variables and a, b and c are constants.

This equation can be expressed in the form y = mx + d where m and d are constants. m is called the slope or gradient. For every increase of 1 unit in x, you will get an increase of m units in y. Also, d is called the intercept and it represents the value of y when x is 0.