Yantrās are pure geometric visual expressions of deities, that make visible, the patterns of sound energy in a mantra. It is a geometrical form that gets created through the various sound resonances of Bīja mantras (primordial sound/seed syllables), through which the Tantra (teaching) and the Mantra (sound resonance) of a deity are interwoven to form a sophisticated design, coded with distinct vibrations. They also represent a more subtle visual form than the anthropomorphic form of a deity.
Yantra literally means a machine/tool. The Sanskrit word 'Yantra' derives from the root 'Yam' meaning to sustain and streamline inherent energies in particular pathways. Hence they act as visual tools that awaken our awareness to the energetic pathways within us, when used in specific ways in conjunction with the specific tantra and mantra. They are created as a visual tool for sadhana (spiritual practices), for moving and opening up specific resonances in the body-mind-spirit continuum, and are always transmitted through a Guru to the disciple.