If f(x) is a function and y=f(x), then y=|f(x)| converts any negative f(x) into positive values, so the graph of y=|f(x)| doesn't go below the x-axis, but is effectively a reflection of f(x) in the x-axis.
For example, f(x)=x3+8. f(x)<0 when x<-2. If f(-3)=-19, so |f(-3)|=19, which is the reflection of -19. The transformation occurs at the point where f(x)=0. In the example, that's the point (-2,0). Instead of dropping below the x-axis the graph would rise above it by the same amount it would have dropped below it (reflection). The positive part of f(x) remains the same.