A credit score is a mark you are given by a lender when it considers your loan application. The higher your score, the greater your chances of being accepted for a loan.
A low credit score will almost certainly preclude you from obtaining credit from a mainstream lender.
Lenders have different algorithms for determining scores, but in general they will examine the contents of your credit report which they access from one of the credit reference agencies in the UK, Experian, Equifax and Callcredit. Your credit report contains a variety of personal information to help lenders avoid identity fraud and determine whether you are creditworthy. It provides a summary of your current debts including details on any loans, mortgages and credit cards you have and whether you have made your repayments on time. It also lists any insolvencies, repossessions or County Court judgments (CCJs) you have registered against you.
Having a number of credit cards where you have reached or are near your credit limits will be cause for concern for lenders as will regularly hitting the bottom of your agreed bank overdraft facility.