One easy way to think about calculus is that it allows us to measure things that are non-linear over time.
If you remember in high school, you would have done things like calculate lengths, areas, and volumes of a variety of objects like squares, rectangles, trapeziums, rhombus', triangles and so on. But you never had the formulas for objects that had curvy parts.
The exceptions were things like circles, and anything involving circles (like a cylinder for example).
Calculus is a way to do this for arbitrary objects, like a parabola, or a circle, or an ellipse, or some other weird object that you have an equation for. You can calculate lengths, areas, and volumes for all of these objects using one framework.
There is more to calculus than this, but this should give you an idea of how calculus generalizes the kinds of stuff you did with linear type objects to objects that are non-linear.