A tankless water heater is far lighter than a tank model since it heats water on demand, but it is wall-mounted and full of plumbing and electronics. The care goes into disconnecting the water, gas, and power cleanly and protecting the connections.
Pro tip: If you live in a hard water area, this is a good time to flush and descale the unit before it is reinstalled.
Watch out: Gas tankless units require proper venting and gas sizing, so reconnection should be done by a licensed pro.