The linear equation is the simplest form of equation you may deal with; it restricts the problem to a unique unknown variable (usually called x) and no exponent (e.g. f(x) = 2x + 15).

The general form of such an equation is: $$\textbf{f(x) = ax + b}$$

Any linear expression can be drawn as a line in a 2D graphic. The graph drawing is straightforward: we have y, or f(x), on the vertical axis (the result) and for a range of x values (horizontal abscissa) we compute and plot the points (x, y).

In brief:
Changing a is changing the slope of the line.
Changing b is changing the offset of the line.