Image: Couple paying bills using laptop © Jose Luis Pelaez Inc, Blend Images, Getty Images

The plea for credit score help was direct, almost desperate. A woman and her husband had placed a "fairly large" good-faith deposit on a home. But then her credit score fell from 626 to 619, just one point shy of the minimum required by the Department of Veterans Affairs to qualify for a VA mortgage.

She already had called to remove two old collections reports from her credit file, but that had failed to give her the boost she needed. Now the couple were in a bind. If they didn't boost her credit score in the coming week, they would lose not just the chance to buy the house but their deposit money as well.

"HELP PLEASE!" the woman, under the user name "Butterflieson," posted on a Web forum run by Fair Isaac, the company that created the FICO credit-scoring model. "Is it possible to raise score 1 point in a few days?"

A step forward, delayed

Millions of Americans have solidly good credit; millions more are stuck with seriously low scores. But there are also millions of consumers in the middle -- people with scores that straddle traditional dividing lines between good and bad. A rise or fall of just a few points could save or cost them thousands of dollars in extra interest payments.

It could even change their ability to buy a house or get a credit card.

"If a consumer is applying for a mortgage and they tend to have higher utilization rates on their credit cards, it could (raise their credit scores and) make a difference" in how much they pay for interest, says Tom Quinn, Credit.com's credit-scoring expert and formerly the vice president of scoring at FICO.

Especially for people caught in the middle, a recent update of the FICO credit-scoring model, called FICO 8, could mean big changes. Depending on the type of credit problems consumers have in their past, some people may receive slightly higher credit scores, which could make it easier to buy a house or get a credit card. For other consumers, the changes will hurt their scores, making it somewhat harder to get credit.

"For individual consumers, it depends," Quinn says. "For some consumers, their FICO 8 score will be higher than their previous score, some consumers will be lower, and others won't change at all."

The Basics I: What is Fair Isaac?

If you don't understand what FICO is and how it works, don't feel bad. The credit-scoring system almost seems designed to be inscrutable to the average person. FICO started life in 1956 as Fair, Isaac and Co., founded by engineer Bill Fair and mathematician Earl Isaac to help department stores and gas station chains decide when to extend in-house credit cards to customers.

Even as the basic scoring model has evolved, the basics have remained essentially the same. A credit score is a number assigned to a consumer that attempts to summarize whether that person is worthy of receiving credit. Mortgage lenders, credit card issuers, car dealers and other businesses that extend credit rely on credit scores to decide whether to extend credit -- and, if so, how much to charge in the form of interest.

Though many companies offer scores, the FICO score has emerged as the benchmark. And FICO itself offers many different types of scores that are customized for different types of loans, including separate scoring models for mortgages, auto loans and even insurance. But they all use similar types of information, including consumers' histories of paying their debts on time, how much of their available credit they use, how many different types of credit they use and whether they have a history of unpaid debts.