A stateless firewall examines each packet in isolation. It doesn't remember previous packets or track connections. Each packet is matched against rules based only on header information.
Stateless firewalls work like ACLs. They check source IP, destination IP, ports, and protocol. If a packet matches a permit rule, it passes. If not, it's dropped.
The limitation: stateless firewalls can't distinguish between a legitimate response and an attack using the same port. You must explicitly allow return traffic.