How is ActiveSupport::SecureRandom secure?

The security of the random number (and its application) comes from how sufficiently random it is for cryptographic purposes. The quality of the randomness relies to a great extent on the entropy that the system can provide such as through interfaces like /dev/urandom on Linux. For it to be secure, an attacker must not be able to guess the next random number, given some sequence of previously generated random numbers.

You could use ActiveRecord::SecureRandom as part of an implementation for creating UUID (version 4) but they are not directly related