An allow list is a security control mechanism that permits access to network resources only for entities on the predefined list, while denying access to all others. This approach is opposite of a deny list, which blocks access for entities on its list but allows all others. An allow list is more restrictive compared to a deny list as it assumes all entities are denied unless explicitly permitted. The other options provided do not perform the function of specifically permitting or denying traffic based on a predefined set of rules, but rather are related to general access management or providing a record of transactions.