Yantra (यन्त्र) is the Sanskrit word for "instrument" or "machine". Much like the word "instrument" itself, it can stand for symbols, processes, automata, machinery or anything that has structure and organization, depending on context.
One usage popular in the west is as symbols or geometric figures. Traditionally such symbols are used in Eastern mysticism to balance the mind or focus it on spiritual concepts. The act of wearing, depicting, enacting and/or concentrating on a yantra is held to have spiritual or astrological or magical benefits in the Tantric traditions of the Indian religions.
Read more about Yantra: Etymology and Meanings, Symbols Employed in Yantras, As An Astrological Device, Philosophical Context