A bug in box is the common name for a box of supplies that will be useful if you ever have to hide out in your own home for a while.
In reality it generally consists of three components.
1) General household supplies.
Most homes have a few days worth of food. In the event of a crisis you should fill every available container with water. You may like to keep an assortment of empty bottles and jars on hand for this purpose. Eat the food in your refrigerator first.
2) Your pocket kit.
This is simply the items you carry in your pocket. This should always include cash, a small torch, two pens and a notebook. Other items are up to you.
3) Bug out bag.
This is a more mobile kit that is intended for outdoor use. Nevertheless it is of almost equal use in the home.
4) The bug in box.
This should contain as a minimum-
A wind up torch and radio.
Potassium iodide-the famous 'anti radiation tablets' of 1950's films. Fairly effective against power station sabotage and dirty bombs.
Potassium permanganate. This is a powerful die and water purifier. It is also a component of explosives so be careful of the authorities. It is also useful for sterilising wounds.
Some good survival books.
Plastic sheeting and elephant tape.
Matches or lighter.
Silver Brittanias (place silver item in water receptacles to keep the water potable).
A good knife.
Plenty of food and more water than you think.