As we say above, every adult has a credit history – which is a record of how well you've managed credit (such as loans, store cards or credit cards) in the past.
Other lenders then use this record when you apply to see if you're a good risk to lend to. Yet if you've made a few mistakes in the past, such as paying late, or missing payments (or even past bankruptcies, defaults and CCJs) then you could appear too high a risk, meaning lenders may not accept you for further credit.
It's also always worth checking your credit reports for any errors as this could be incorrectly damaging your rating (and may impact which cards you can get). See 28 tips to improve your credit score for full help.