Live Location will share your current location with friend or group of friends on Facebook Messenger for one hour, after which you go back off the grid until you decide to share again.
To use Live Location, open a conversation in Facebook Messenger. If the location icon is visible next to the text field, tap on it. If not, tap on the "+" sign to the left of the text field, then Location.
Those who have the new feature will then see a map with your current location, along with a big blue button titled Share Live Location. Tap on the button, and approve the app's request for access to your location data when in the background if prompted.
Participants in the conversation will then receive an attachment with your current location and the option to get directions to your whereabouts. A timer is also shown, letting you and them know just how much longer you can be found.
After the hour is up, your location goes back to private. Alternatively, you can stop sharing your location whenever you want by tapping on the attachment and then Stop Sharing.
The feature is slowly rolling out to users, so if you don't have it yet, keep trying. It took a few hours after Facebook's announcement before I had access.