Short answer: Imagine that the integer used in the for loop is a float instead.

Longer, a bit more precise answer: An integer can only have discrete values (i.e. -1, 0, 1, 2, …, 69, … etc.)

A real number (~float with infinite precision) can have an infinite amount of values between two discrete values.

An integral is, to put it simpy, a sum of all the results of taking those infinite values between two discrete values (an interval) and feeding them to the given function.

It’s a for loop over an infinite set of real numbers rather than over a finite set of integers => a non-discrete for loop