Many very good poker players cannot manage their money.
Some players waste their money gambling on other activities where they do not have the same edge they do in poker.
Other players blow all their winnings playing over where their bankroll allows.
Bankroll management is a concept which helps to restrict the latter.
A conservative perspective on bankroll management, dictates you want to have at least 30 buyins for your game of choice.
So at NL50, you would want to have $1,500 in the bank, and at NL2000 you would want $60,000.
Moderate perspective is 20-25 buyins.
The lowest you would want to take a shot at is 15, but your risk is very high here.
No one wants to lose their buyin when they are over their heads in skill and money.
If you are a new player, multitable a lot, play 6max, or play a very LAG (Loose Aggressive) game, the number of buyins should be higher, not lower.
Another tip, do not play with scared money.
Do not buy into a game where you are worried you are going to get killed in.
Or one that is a stake way above your bankroll just to get a quick win.
Do not sit down at a table with the minimum buyin while scared, thinking this will minimize your risk.
You will be playing against players better than you are used to and stakes you have little experience in.
Also, do not play with rent money, or money for school, or a car payment.
This is similar to scared money.
Your play will be worse because more is riding on it.
Play at a game where if you lose, it does not matter.
Having financial 'invulnerability' will turn you into a fearless player, and a much better player.