A pendulum is a short string or chain with a weight at one end. The weight can be made of almost anything, but crystals, various polished stones and metal are common. It’s length is approximately 6 1/2 inches or 16 cm but can vary according to personal preference. They may be purchased in stores or on the web, or you can make one by simply tying a piece of string to a small object. A couple online resources:
Amazon
Etsy

Historically the pendulum is associated with divination. Divination is a term that refers to gaining insight to a question by means of a particular process or ritual. The methods vary but have in common not using the conscious mind to derive the answer.

Most of the time we answer questions with our conscious mind by reasoning and memory. But we also get information through intuition, dreams, and creative ideas that pop into our head. What these have in common is that they emerge into consciousness from some unconscious source without our conscious cogitation.

By using a pendulum, we can encourage unconscious information to become known through a process of asking questions and witnessing the answers through the spontaneous (i.e unconsciously induced) movement that occurs.

We begin by training our body to move the pendulum. Once the body can control the pendulum without deliberate instructions, we can instruct our subconscious mind to move the pendulum and answer a question that we pose to it. The conscious mind is put to rest and just observes to see what the subconscious produces.