Let us define the function f(x) = \boldsymbol{w}^T\boldsymbol{x}+b = 0, as the function that determines the line L that separates two classes (our two features), see the figure here.
Any point defined by \boldsymbol{x}_i and \boldsymbol{x}_2 on the line L will satisfy \boldsymbol{w}^T(\boldsymbol{x}_1-\boldsymbol{x}_2)=0 .
The signed distance \delta from any point defined by a vector \boldsymbol{x} and a point \boldsymbol{x}_0 on the line L is then \delta = \frac{1}{\vert\vert \boldsymbol{w}\vert\vert}(\boldsymbol{w}^T\boldsymbol{x}+b).