The horizontal bar in the radical sign tells how much of the expression is the radicand. In this expression: the bar stretches across the whole expression p - q. So this means we subtract q from p, then find the square root of the result. The whole expression "p-q" is the radicand.
If it has only covered the 'p' like this: it would mean find the square root of p then subtract q from the result. Only "p" is the radicand.
The bar acts like parentheses, telling you how to group items and the order of calculation.