Gradient tells you how steep a straight line is. It compares how much the line goes up or down with how much it goes across.

From left to right, a line that goes up has a positive gradient and a line that goes down has a negative gradient. The bigger the size of the gradient, the steeper the line.

The key idea is rise over run.

$$m=\frac{change\; iny}{change\; inx}$$

Here $m$ means gradient, with the change in $y$ on top and the change in $x$ on the bottom.