social security number
hi hakigo - i think you mean SSN - as in social security number.
when you apply for a credit card, car loan, bank loan, etc., they always ask to see your social security number because this is a number that credit bureaus use to track whether or not in the past you have been a good customer. they can check whether in the past you have paid your bills on time by entering your social security number into a computer that checks your "credit history" or "credit rating."
just having a social security number might not be enough to get a car loan, credit card, etc. - because even though you now have a SSN - they will type it into a computer, and the computer will show that it has no information about how loyal you have been in the past about paying your bills. Your number is brand new! Most Americans have had their SSN since their were born, and have a long credit history by the time they start applying for car loans and mortgages when they are in their 20s or 30s. This way, they know if you are a "bad" customer or a "good" customer.
since you will have no history when they plug in your number into the computer - they might not want to risk giving you a loan or anything else because you might not pay the money back.
these days, however, credit card companies seem very happy to give people credit quickly. you can probably get a credit card very easily, but they may start out by giving you a very small credit - like only 2,000 dollars or something.
For a car loan or bank loan, they will probably still negotiate with you even if you have no credit history if you give them proof of employment and salary.
hope that helps,
nora