It’s a company that gives you access to a service, and will need your account information to provide it.
Third party providers are approved by the Financial Conduct Authority before they can take part in open banking. All third parties will appear in the open banking directory.
Authorised third party providers could be high street banks, smaller banks, or companies that aren’t banks such as credit reference agencies or comparison websites.
You can find more information and a current list of providers on the dedicated open banking website https://www.openbanking.org.uk/