Mosquitto is an open source broker for MQTT. It’s versatile and supported on a wide range of platforms. MQTT is a pub sub protocol that uses a broker as the central source for registration.
MQTT supports a quality of service which is used to specify how resiliant a message is - it can be guaranteed like TCP, or can be “best effort” in which there is no guaranteee (like UDP).
Recently I got a raspberry pi nano 2W which I can use as an MQTT client, and can run the broker on a higher power machine (e.g. my dev machine or one of the pi 5s).
Put together an example application that uses MQTT to get a better idea of how it would be used in an internet of things (IoT) based setting. Maybe compare and contrast this with another protocol such as zenoh which is also pub/sub, but does not have any network overhead and can have geo-distributed storage.