The GainNode is a component of the Web Audio API, which is used in web development to manage audio signals. It allows developers to control the volume of audio playback by adjusting the gain, or amplification, of the audio signal. This makes it easier to create dynamic audio experiences in web applications. GainNodes can be connected to other audio nodes, such as AudioBufferSourceNode or MediaElementAudioSourceNode, to manipulate sound in various ways. By changing the gain value, developers can create effects like fading in or out, making it a versatile tool for enhancing audio in websites and applications.