The MediaElementAudioSourceNode is a part of the Web Audio API that allows developers to play audio from HTML media elements, such as audio or video tags. It acts as a bridge between these media elements and the audio processing capabilities of the Web Audio API, enabling advanced audio manipulation and effects. By using the MediaElementAudioSourceNode, developers can control audio playback, apply filters, and create spatial audio experiences. This node is particularly useful for web applications that require synchronized audio and video playback, enhancing the overall user experience with rich sound design.