First, update your package lists, install Mosquitto broker and clients, enable, and start the service.

sudo apt update
sudo apt install mosquitto mosquitto-clients
sudo systemctl enable mosquitto
sudo systemctl start mosquitto

Terminal 1 (subscriber)

In one terminal window, subscribe to a topic to listen for messages.

mosquitto_sub -h localhost -t "test/topic"

Terminal 2 (publisher)

In a second terminal window, publish a message to the topic.

mosquitto_pub -h localhost -t "test/topic" -m "Hello World"

You should see "Hello World" appear in Terminal 1.