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