A Container Network Interface (CNI) allows users to configure networks and interfaces in the cluster (see https://kubernetes.io/docs/concepts/extend-kubernetes/compute-storage-net/network-plugins/). More specifically, it is how you add an interface backed by a Solarflare NIC to your pods. This is necessary because, by default, the interfaces in pods are software-defined and cannot be accelerated.
There are many CNI plugins, but Onload Operator for Kubernetes v3.0 supports only the Multus CNI (see https://github.com/k8snetworkplumbingwg/multus-cni).
You must configure the CNI yourself, depending on your cluster, topology, and configuration. You must provide a Network Attachment Definition (NAD) kind of CR stating the master interface (to select the Solarflare NICs) and a range of IP addresses.