What is the explanation for rubeque prime factors?

What is the explanation for rubeque prime factors?

So this is my code, the thing that is bugging me is this line " { |i| (num
% i) == 0}.length == 0? "can someone eloborate calling .length on a block?
I am trying to answer a rubeque problem prime factors
divisors = Array.new
for d in 2..(num)
divisors << d if (num % d) == 0
end
primes = divisors.select do |num|
(2..(num-1)).select { |i| (num % i) == 0}.length == 0
end