Calculus - specifically integration with limits.
Used for calculating the volume inside a non-linear shape.
As the limits are pi/2 and -p/2 ...pi is radians not 3.141 ...its clearly the volume inside a 360 degree shape - something like an arced tube.
As the final upper limit is 'bwL' the answer cannot just be a number, using that screenshot.
I suspect the text book then moves into the rate of flow of a fluid through this shape and uses differentiation, another branch of calculus.