API keys are one of the primary components of API authentication; they are the API equivalent of your Kraken account's username and password.
API keys are required to call any of the account specific (aka private) API endpoints, namely the account management, trading, funding, and earn endpoints. API keys are not required to call the market data (aka public) API endpoints, as the market data endpoints are not associated with any specific Kraken account.
API keys can be used by any software that interacts with our markets and accounts, such as our REST API command line clients, some mobile apps, third party trading bots, and many other products/services.