Rudraksha is a dried fruit of a tree that grows in Nepal, Himalayan region of India and some parts Indonesia. Rudraksha beads are worshiped or worn from time immemorial to gain mental peace, spiritual and material benefits. Botanically, Rudraksha plant is called Elaeocarpus Ganitrus Roxb. Majority(almost 75%) of Rudraksha trees grow in Indonesia, the rest grow in Himalayan region(25% in Nepal and 5% in India).
Rudraksha is a Sanskrit word formed by joining two words: Rudra + Aksha. Rudra means Hindu God Lord Shiva and Aksha means eye/tears.
According to Hindu scripture, Shiva Purana, Lord Shiva opened his eyes after several years of meditation. From his lotus-like eyes, compassionate tears fell on earth and they gave birth to rudraksha trees.
From perspective of Tantra, Rudraksha means Shiva and Shakti. Rudra means Shiva and Aksha mean all letters of Sanskrit alphabet( A to Ksha) representing Shakti, i.e. Parvati.
Shakti is all-pervading. She is present in animate beings and inanimate objects like Rudraksha. If we hold rudraksha bead in our hands, we can feel natural spanda or vibrations. These vibrations represent dynamic form of Lord Shiva i.e. Shakti/Shridevi. So, Rudraksha bead itself is Rudra/Shiva(static form) and the vibrations are Aksha i.e. Shridevi(dynamic form).
Moreover, Rudraksha is a great example of inseparable nature of Shiva and Shakti. As great poet Kalidasa puts it Shiva and Shakti are intricately united like vagartha (word and its meaning). In Saundaryalahari, Our primal Guru, Shri Shankara Bhagavatpadah says:
Shivah shaktya yukto yadi bhavati shaktah prabhavitum … | – Saundaryalahari, Shloka #1
i.e. Shiva can act only by uniting with Shakti(i.e. consort Parvati). As explained, in Rudraksha, Shiva(Rudraksha) is always united with Shakti(vibrations), so, by definition, rudraksha is naturally powerful and is an epitome of oneness of Shiva and Shakti!
Om Namah Shivaya ||