Since the Echo and Google Home (smart home assistants) are unable to communicate directly with most smart home devices, you can install Axial Server on a home computer to be a "bridge" between your smart home assistant and your smart devices.
If you use z-wave smart home devices, you will need a special USB stick that speaks the z-wave language; this will enable it to communicate with your devices.
Once Axial Server is setup and you have activated the Smart Home Assistant plugin, your home computer will communicate with the Echo or Google Home to enable control of your smart home devices.