An indirect-fired water heater is essentially a water storage tank that is heated through the boiler. Because of this, it relies on your home's boiler to be high in temperature, so that it heats up the water within the indirect water heater.
Here's a diagram to illustrate how this actually works.
These kinds of water heaters, or indirect water storage tanks are best used for larger homes, where the boiler is constantly being used, hence it will be consistently heating the water within the storage tanks.