There isn't really an objective except to survive. You free roam an open world with towns and villages separated by woods and trails. You loot different weapons, gear, food, bandages, etc. There are zombies and they are fast. You can join servers with up to 16 other people or make private ones with friends. You can choose whether you want pvp or not. Pvp is actually pretty thrilling. But riding around with friends is too. There are different vehicles you can find and drive which makes things a lot faster than on foot.
There's a day/night cycle.
Pretty sure I've heard it's still an alpha version but as a free phone game it's a load of fun.