This is a security feature on access control, it is a control implemented in the access systems to prevent a person that just entered a controlled area from giving his/her Card or tag to another person or using their fingerprint to allow them access to that particular area. Without this control more people than the authorized could access a controlled area with one card or tag.
The benefits of Anti-passback:
It prevents the same tag, card or fingerprint from being used more than once to gain entry to a controlled area, unless they clock out first.
It prevents a tag, card or fingerprint from being used until the system detects a pass from the opposite side, an entry will not be allowed until an exit is detected.
To set up Anti-passback, one can do it on the device but the setup differs with each device. You can also do the setup using a tool if the devices are connected to a controller (Contact Binary City for assistance).
The screenshots below illustrates how the Anti-passback setup works.