This is a basic question. Distance is defined mathematically by the square root of the squared difference between a and b. Then what do the expressions a-b and Ia-bI (absolute value) mean?
The distance between two points $\displaystyle a~\&~b$ on a number line is $\displaystyle \left| {a - b} \right|$.
The distance between two points in a plane $\displaystyle (p,q)~\&~(r,s)$ is$\displaystyle \sqrt {\left( {p - r} \right)^2 + \left( {q - s} \right)^2 } $.
Does that help?