First, let's work out the equation of the graph.
We can see that for every 1 in x, the y coordinate increases by 1, which means the gradient is 1. The line intercepts the y axis at -1, meaning the equation of our line is y = x - 1
An inequality in this case basically represents which side of the line we want to be on - do we want the numbers to be greater than the line (i.e. above the y value) or less (i.e. below the y value)?
The shading here shows us they want the value set below the line, i.e. less than it.
Therefore, our inequality is y ≤ x-1