Your credit utilisation is how much of your total available credit you’re using across all your credit cards, shown as a percentage.
For example, if you have one credit card with a £1000 limit and £100 balance, your utilisation will be 10%. Or if you have two cards, each with a £1000 limit and a balance of £200 across both, your utilisation will also be 10%. This percentage is not affected by your instalment loans (for example, if you have a mortgage).
If you keep your utilisation low and spread evenly across all your cards, your credit score will improve. A low percentage shows you're using less of the credit available to you.
Please note that your credit utilisation is one of the more influential factors to your credit score, but it’s not the only thing that can affect it.