Reflexive access lists allow you to dynamically open up your filtering router to allow reply packets back through, in response to an outbound TCP connection or UDP session initiated from within your network.
This is exactly what the ASA's stateful inspection does by default. It allows traffic from a higher security level (inside interface) to a lower security level (outside interface) and only lets traffic from the lower security level interface to a higher security level interface (from outside to inside) if it's part of a response to an outbound request -- of if the traffic is explicitly permitted inbound on an ACL.
Hope that helps.
