yup thts th rule used in sin cos and tan curves. i could not understand what he meant.Simply draw the graph of the original equation. Now you will see the curve has a minimum point, because the coefficient of x^2 is positive. This minimum point (turning point) if exists below x-axis, ie. in -ve y-value, reflect that part above the x-axis, taking y=o as the reference line for reflection.
This is what modulus does. It always changes the negative y-values to +ve.