The idea is simple: you select specific people in your address book as "trusted contacts," friends or family members who you would feel confident knowing your phone's activity status. When they open the app, they'll be able to see if you're "active," basically that your phone is connected and moving, whether you were active in the past hour or whether your device has a low battery, is completely out of juice or has no connectivity.
Trusted contacts can also ask for your location if they feel you might be in danger. The app offers a five-minute window for you to approve or deny the request but should it not receive a response, it will share your position in order to "make sure that someone you trust will know how to find you if you really are in trouble."
If you're walking home late at night and would feel safer knowing that someone knew exactly where you are, Trusted Contacts can help there too. Instead of waiting for someone to request your whereabouts, you can proactively share your location with a friend or loved one and include a short status explaining what it is you're doing. Once you arrive at your destination, hit the banner top of the screen or from the lockscreen and it'll stop broadcasting.
Google says that while trusted contacts won't need a Google account to see shared locations, they may need to sign in with one to request them. The app's settings can also be tweaked at any time, allowing users to decide who they do and don't want seeing their activity without having to re-add them at a later date.