If in range, you can attack other players reducing their health or armour.
To refill your health and armour or to purchase other items and weapons, there will be an ingame store where you can spend your earned credits (which will not be a value of real money).
As long as you are outside of one of your secret hideouts, your last known position will always be visible on the map and you are attackable.
To be protected, you will have to enter one of your safe houses.
As you might understand, this game needs its players to come to life. So if there's no other player around your location, you won't be able to fully take advantage of all functions.
In this case, best recruit your friends to bring some movement into the game.