Sidekick is a tool that offers the functionality of ChatGPT directly at user's fingertips on MacOS systems. It serves as an interface to facilitate interaction with OpenAI's ChatGPT, a conversational AI model.

The tool is designed to function similar to the Spotlight feature on MacOS, but specifically focused on providing access to conversational AI abilities.

The Sidekick tool requires an OpenAI API key to operate, indicating that it uses OpenAI's API for interaction with the chatbot model. While the tool is provided free of charge, it allows users to contribute or 'donate' as part of its model.

The tool emphasizes on privacy, assuring users that their data would be secure. Sidekick is developed by an individual developer named August Wester maintaining user privacy and utility at its core.

Users should note that as an interface between the user and ChatGPT, Sidekick's functionality and features would be dependent on the capabilities and limitations of the underlying ChatGPT model.