A usage plan specifies who can access one or more deployed API stages and methods—and optionally sets the target request rate to start throttling requests. The plan uses API keys to identify API clients and who can access the associated API stages for each key.

API keys are alphanumeric string values that you distribute to application developer customers to grant access to your API. You can use API keys together with Lambda authorizers, IAM roles, or Amazon Cognito to control access to your APIs. API Gateway can generate API keys on your behalf, or you can import them from a CSV file. You can generate an API key in API Gateway, or import it into API Gateway from an external source. For more information, see Set up API keys using the API Gateway console.

An API key has a name and a value. (The terms "API key" and "API key value" are often used interchangeably.) The name cannot exceed 1024 characters. The value is an alphanumeric string between 20 and 128 characters, for example, apikey1234abcdefghij0123456789.

https://docs.aws.amazon.com/apigateway/latest/developerguide/api-gateway-api-usage-plans.html