Stateful filtering is a feature that monitors traffic originating from one zone and moving to another. It keeps track of this information and would allow only the returning traffic through the ASA.

Traffic originating from a zone with a higher security level, such as the INSIDE zone (100), is allowed to go to a zone with a lower security level, such as the OUTSIDE (0) and DMZ (50), and the return traffic is allowed because the ASA keeps track of the flows of traffic in a state table (stateful filtering):

However, traffic originating from the OUTSIDE (0) zone is not allowed to reach the INSIDE (100) zone or the DMZ (50) by default, nor is traffic originating from the DMZ (50) allowed to access the INSIDE (100) zone.