The Genius API is a web service that allows to programmatically read/write song lyrics (and their annotations) via HTTP.
The Genius website and app provide a dataset of song lyrics that aim to explain the meaning of songs. The Genius API is a web service provided by the authors of Genius that gives programmatic access to part of this dataset by making HTTP requests.
An access token is required to use the API. The API's documentation documentation gives more details.