Once Brahma was disheartened because he tried many times different kinds of srishtis but was not successful. So he started doing penance for Shiva. Soon Parbramha presented himself along with Maa Shakti as Ardhanarishvara . The manifestation of Ardhanarishvara came into existence. Brahma prostrated them and prayed to them by chanting Ardhanarishwara Stotram.
Pleased by his devotion, Maa Shakti and Shiva granted him the boon to create a universe with male and female species. And hence the whole universe came into existence. Ardhanarishwara is a unique form of God signifying that male and female are equal and incomplete without each other, none is superior to the other, and hence both shall respect each other. The Tantra elaborates them in detail and has methods for worshiping Ardhanarishwara to gain many siddhis ,wishes and moksha. This aspect also signifies unity of ShivaShakti.
Parmatma and his chichakti are one like the sun and its light. Shivahriday Uapnishad describes it beautifully in verses. Shiva and Gauri, They are one, inseparably one. His heart wouldn’t beat without Gauri. He couldn’t breathe without Gauri. He wouldn’t survive a day, a moment, without her love.He needs her, as the fish in the sea needs water, as the birds in the sky need air. If he is the day, she is his light. If he is the night, she is his coolness. He can’t exist without her. Shiva looked at his beloved and said, “No more shall you suffer any pain. From today, you and I shall not be two bodies and one heart, but a single united body, a single heart that beats with our love, a single soul glowing in that love”.Shiva gathers Gauri in his arms and holds her so tightly to him that the two unite and became one.