Essentially the concept of a bug out bag is for when you need to leave in an hurry and you want to grab everything you need all at once. So instead of spending an hour looking for things and stuffing everything into your bag, you have a prepared bag which is ready to go - which you just grab and go immediately.
The suggested contents of a bug out bag vary depending on the website or group which prepared the list, but the commonly quoted items are as follows...
Non-perishable food for 72 hours.
Water purification and sanitation supplies (toilet paper, etc).
A first aid kit.
Fire starting tool (eg. matches, ferrocerium rod, lighter, etc).
A pocket survival guide.
Weather appropriate clothing (eg. poncho, headwear, gloves, etc).
Sleeping bag and blankets.
Any medicine you require for 72 hours or more.
Battery or crank-operated radio.
Flashlight (crank or batteries) or glow sticks.
Cash and change.
Positive identification, such as passport, birth certificate, drivers license or social security card.
Fixed-blade and / or folding knife.
Small tool kit.
Duct tape, rope and / or fishing line.
Plastic tarps for shelter and water collection.
Optional Weapon / Hunting / Fishing Gear (eg. slingshot, pellet gun, firearms + ammo, archery equipment, crossbow, etc).