Yantra 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

Eastern mysticism

Eastern mysticism or Eastern spirituality is a broad and largely Western concept summarizing and sometimes amalgamating mystic traditions of the Middle East, the Indian Subcontinent and the Far East, a separate realm from Western mysticism...

 to balance the mind or focus it on spiritual concepts.