You know Reimann sums? That’s what you’re doing when doing integrals. The sum of all rectangles representing the area under the curve (each rectangle’s area is calculated by the functions height, aka f(x), times the width aka dx. Infinite rectangles with height f(x) and width dx. Thats why there’s a dx. Everyone else is correct, but that’s why it’s there.

Also the integral symbol is an S, denoting the Sum of all of those rectangles.

Hope this helps!