Replacing a water heater is really a series of steps that ultimately lead to you taking a hot shower. When you remove a water heater, you drain it and install a new tank (assuming you are using a tank-style storage heater). That tank is full of air and you need to bleed the air out of the system.
Once the new water heater is connected to the piping, you turn the water back on and start refilling it. The air in the tank now gets displaced by the water filling it. So as the water enters the tank, it pushes out the air. The air has to go somewhere, so you need to open a hot water fixture close to the tank so the air can exit the system. You will hear the air hissing as it comes out of the faucet. When water finally starts coming out of the faucet, the tank is full of water and you can shut off the faucet. Now, this doesn\’92t mean that there is no air still left in the system. You will likely get a few burps of air when you open different faucets within the house. Once all the air is purged, you won\’92t get the burping anymore.