Experian are one of the primary credit reference agencies in the UK.
When you want to take out a credit agreement - loans, mortgages, credit cards, phone contracts etc. - the lender needs to assess the likelihood that you will be able to meet the payment obligations that come with that agreement. They do this by looking at your history of successfully meeting other payment obligations, which is stored in your credit file by credit agencies such as Experian. Credit agencies aggregate up your financial history into a credit ‘score’ which lenders use as an indication as to how much credit you can successfully manage. The higher your credit score is, the more credit opportunities you have available at better terms (i.e. at lower interest rates).
For more information on Experian see their website https://www.experian.co.uk/