You need to give us a bit more info, that's a pretty broad question. Are you having a problem? Very basically your propane tank feeds both hot water and furnace. When the thermostat calls for heat, the propane valve opens and the igniter ignites it. Runs until the thermostat says enough. There are lots of other parts and failsafes involved. It's a big help if you would put your location and rigs in your signature. Welcome to the forum.
2016 Arctic Fox 27-5L
2018 GMC 3500 Duramax