The differential. It's an operator meaning "infinitely small slice of ..." Since the Integral symbol represents an infinite sum, Integral( (f(x)*dx) ) is an in

The differential. It's an operator meaning "infinitely small slice of ..." Since the Integral symbol represents an infinite sum, Integral( (f(x)*dx) ) is an infinite sum of infinitely small x-wise slices of f(x), thus giving you the area under the curve f(x).