What is the meaning of k in this function?

[buran]

of course k is an instance of range(2, n)...

no, it's not an instance of range object. because of the loop, it takes the values (int), produced by range, one by one in each iteration of the loop.
you can visualise the execution here: http://www.pythontutor.com/visualize.html

compare

def smallest_factor(n):
    numbers = range(2, n)
    print(f'numbers is {type(numbers)}')
    for k in numbers:
        print(f'k is {type(k)}')
        if n % k == 0:
            return k
smallest_factor(33)