Multicast DNS.
This can be a network killer, and something to consider when performance isn't great on your network.
Newer operating systems and browsers including apple have enabled mDNS reponder functionality so networks can be quite chatty and consequently generate large amounts of traffic. Being UDP (connectionless) mDNS traffic can directly affect necessary services on your network, including DNS and more importantly DHCP.
Bonjour enbled devices like printers, AppleTV, Google cast, Google browser announcements use mDNS for service advertisements and streaming services.
If you are running an apple devices in large numbers on the network or utilise google casting and wonder why things aren't working so great then it may be due to excessive mDNS traffic.
We can advise on best practise to ensure critical services aren't affected and how to manage this kind or traffic at scale.