Bankroll Management !

To understand why Bankroll Managment is so important, you have to understand how you should approach the game of poker.

Poker is a long term game, it is not AA v KK getting cracked on the river. It is a game of thousands of iterations of these hands. Throughout the carreer of a poker player, you will see aces more than your fare share and though you will lose with them from time to time, you will win a lot more. Everytime you play aces properly, it is a positive investment even though you may lose money on any given occurence. It's not a game of, "Mah gaht dahm aces got cracked a-gaht-damn-gain! Gah!"
Poker is a horrible short term game, you can be ontop of the world one day and drowning in the depths of the river valley the next. It's a game of swings. You WILL lose at some point, it's a statistical certainty. It is the undefinable "luck" that people are so quick to blame. So you need to make sure you have enough money in your online bank so that the loses don't phase you. You need a fat enough roll to never go bust becuase of a bad streak and to help prevent tilt.100 bucks just isn't what it used to be from a poker perspective.
The numbers might be a little off here or there but the idea is: If you're never risking much of your bankroll from day to day, you allow yourself to play your game risk-free. Losing money won't drive you to suicide *cough* And your back should never be against the wall.

Poker is a game of positive longterm investments, you want to make sure that you have enough money to stick around and reap the rewards of these investments.