These distinctions are common in metro area networking were an ISP needs to keep track of their uplink and downlink ports.
NNI ports = Network Interface Port = Uplink port = WAN ports.
NNI port characteristics include being faster than UNI ports, being a high speed “highway” for aggregated traffic across all of the uni ports, it functions as the uplink/wan port and is often modular and used with SFPs (small formfactor pluggable transceivers). There exists many different types of NNI/uplink ports, with different form-factors and speeds.
UNI ports = User Network Interface = Downlink port = LAN ports.
UNI port characteristics include being slower than the NNI port, there are more UNI ports on switches than NNI ports (for example 24x10g UNI ports vs 4xNNI 25g ports). These ports are either copper or fiber based ports. Fiber ports use SFPs for modularity.