Thirumanthiram -- Tantra 1:

The One is He, the Two His sweet Grace, 

In Three He stood, in all the Four witnessed, 

The Five He conquered, the Six He filled, 

The Seven Worlds pervades, manifests the Eight and so remains. 

~Saint Tirumuoolar

•From - Tirumantiram, Tantra 1, Verse 1

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Brief Explanation :-

1. The One—The Uncreated Eternal Being—Sivam. 

2. The Two—Siva/Sakti. Sakti is the dynamic aspect of static Sivam. She confers grace on the Jivas. 

3. The Three—Triads are many: 

a) Primal Sakti evolves into three subsidiary Saktis: Ichcha (desire); Jnana (knowledge), and Kriya (action). 

b) The three Gods: Brahma or Aya (of creation), Vishnu or Mal (of preservation), and Siva or Rudra (of dissolution). 

c) The entire creation represented by the three genders: he, she and it. 

d) The three theological categories Pati, pasu and pasam. 

4) The Four— 

a) The four Vedas: Rig, Yajur, Sama, Atharvana.

b) The four steps to God-realization: Charya and Kriya, Yoga and Jnana. 

5) The Five— 

a) The five senses. 

b) The five acts: creation, preservation, dissolution, obscuration, and liberation. 

6) The Six— 

The six adharas or plexuses (chakras): i) muladhara, ii) svadhishthana, iii) manipuraka, iv) anahata, v) visuddhi, vi) ajna. 

7) The Seven— 

a) Seven Worlds: Bhuloka, Bhuvarloka, Svarloka, Maharloka, Tapoloka, Satyaloka, and Sivaloka. For an elaborate and esoteric account of the lokas (worlds), see the Vishnu, Bhagavata, Vayu and other Puranas. 

b) The seventh chakra—sahasrara located in the cranium. 

8) The Eight—Pervasiveness through earth, water, fire, air, sky, sun, moon and Jiva.