A credit score is a number lenders use to decide how risky you are to lend to — and therefore how much extra to charge you. Same $20,000 car, two buyers, same price: the one with good credit pays thousands less over the loan, because their interest rate is lower. Your score is a reputation you're building whether you're paying attention or not, and landlords and insurers peek at it too.
The big levers are paying on time (the single most important factor) and not maxing out your cards (keeping balances low relative to your limits). Length of credit history and not opening too many new accounts at once matter too. The fixes are insultingly boring: pay on time, keep balances low, and wait.
More than people expect. A lower score means higher interest on car loans, mortgages, and credit cards — sometimes thousands of dollars more for the identical purchase. It can also mean bigger deposits, worse insurance rates, and rejected rental applications.
Pay every bill on time, keep credit card balances well below their limits, avoid opening lots of new accounts at once, and let time do the unglamorous part. There's no clever trick — just consistency that compounds into a better number.