This is the rate at which the line is increasing/decreasing
To find the gradient of a line two points on the line a required. Between these two points you find the change in height (rise), and the change in the distance traveled horizontally (run).
In the example on the right you can see the rise of 4 and run of 2
Therefore for this example the gradient is two. This means that for every value that x increases y will increase by 2.
Gradients can also be negative (going down as opposed to up), and don't always have a to be whole numbers.
Take a look at the example here here the line is going down one between the two points but it is two values across between the points. Therefore the gradient is as follows: