Motion detection is a common feature in home security cameras, but it isn't really included in Nest devices, though, because Google has announced a new version that will enhance voice recognition on its Nest speakers and displays.
This means that your Nest speakers or displays will be able to detect your presence either via voice or touch. Right now, smart speakers and displays only activate when the user presses a voice command, but by incorporating presence detection, these devices will be able to recognize when you're there and act accordingly.
If you were to enter your house, your smart speaker or display might detect your voice and notify you that you're there, and may then automate certain actions, such as setting the temperature higher or lower (depending on the season).
In some instances, Google's Soli radar-sensing technology and sensor will be used for presence detection; however, not all Google's Nest devices have it, and presence sensing will differ from device to device. Click here to learn more about how each Nest device might be able to detect your presence.