What it's for
Port 161 is the port SNMP (Simple Network Management Protocol) agents listen on for queries from a network management system. It's the backbone of a lot of traditional network monitoring — polling routers, switches, printers, and other infrastructure devices for status metrics (interface traffic, CPU load, error counts) without needing a dedicated management protocol per vendor.
Protocol basics
SNMP uses UDP for its request/response queries (GET, GETNEXT, SET operations against a device's MIB — Management Information Base — tree of values). Older SNMP versions (v1 and v2c) authenticate using a simple "community string," which is transmitted in plaintext and functions more like a shared password than real authentication — a well-known weakness. SNMPv3 addresses this with proper authentication and encryption, and is the version that should be used for anything beyond an isolated, trusted management network.