The Padlock is a new item (along with the Suitcase) introduced in the "What's in the suitcase?" September 2015 DLC.
The Padlock appears to be a red lock. There's a key included when in the shelter.
During scavenging, the padlock takes up 1 inventory slot. It is usually found hanging on a wall or attached to a cabinet in one of the bedrooms.
The Padlock has three major uses in gameplay. When the family is attacked at all, it is one of the options they can select to drive away the attack. It can also be taken outside during an expedition, and its use can allow the player to get 2 Water Bottles. Finally, if the Padlock is in the shelter and there is only one character left alive in gameplay, it allows for one final expedition by that single person--the game will automatically skip ahead the several days to the end of the expedition.
The Padlock has only one use and cannot be repaired; the dialogue suggests that it takes a considerable amount of damage from raiders' attempts to break it. Once it is unlocked, it is damaged beyond repair and is simply disposed of.
- It is one of the two defense items that will always get consumed/destroyed upon using them. The other one is the Bug Spray.