The network policy works as a software firewall to the pods. By default, every pod can communicate with each other without any boundaries. The network policy is one of the isolations you could apply to these pods. This defines who can access which pods in which port by namespace selector and pod selector. The network policy in a namespace is additive, and once a pod enables the network policy, it denies any other ingress (also known as deny all).
Currently, there are multiple network providers that support the network policy, such as Calico (https://www.projectcalico.org/calico-network-policy-comes-to-kubernetes/), Romana (https://github.com/romana/romana), Weave Net (https://www.weave.works/docs/net/latest/kube-addon/#npc ...
