typist, a FICO score is used by a lender for risk analysis, when considering you for a line of credit. And the only way that can be measured, is by empirical data accumulated by the CRA's on your history of credit(your credit file).
Data is stored on you for 7 years regarding how you handled your various credit instruments.
If you've had no history of credit in the past 7 years, you have a "thin" file(very little information that the credit grantors use to acess your credit worthiness). No debt over an extended period of time = not enough info to accurately measure you against the historical database they have.
So the trick is to have credit extended to you, and manage what you owe to the credit grantors wisely.
I know that it as clear now, as six feet of muddy water. That's how I felt when I came here. But through your own research and time spent reading on this forum, you'll get a firm grasp of the workings.
Hope that helped. Good luck to you. 