The Mool Mantar is the most pure concise description of existence (consciousness). Mool means your root or source, so it is the root mantra by hearing or reciting to anchor in your source.
It is the opening verse of the Sri Guru Granth Sahib. The core message of the 10 Sikh gurus is 'apna Mool pashano' (realize your Self) by purifying the mind into silence through meditation, truth, devotion or service.