In plain words
“O Rudra, that form of Yours which is auspicious, not terrible, and destroys sin!” Prajāpati, the Puruṣa of the Vedas, has two kinds of form and nature, of shape and feature. The terrible form is named Rudram, and the one that is not terrible, Śivam. When Narasiṃha split the pillar and came forth, Prahlāda saw that beautiful form and was beside himself with Ānanda; to Hiraṇyakaśipu, the enemy of God, it appeared as a fierce form and filled him with fear and dread. That is to say, it becomes clear that within the one Divinity itself the fierce and the gentle, the terrible and the benign, are seen according to the cast of a man's mind.
Not only this; these two forms themselves are named as the ground of creation, sustenance and dissolution. For the welfare of the world, that Paramātma Himself is showing His own form and nature as Śivam and as Rudram.
The terrible and the benign are quite common in the world. When you eat things that are there to be enjoyed, the contentment, the nourishment, the satisfaction and the health that come of it are the benign, the auspicious. Eat those same things beyond measure, and they take on a terrible form and carry you to the very brink of death! That is to say, the things, experiences, situations and events that stand in the way of Ānanda are all terrible; moderate eating that keeps within its limits gives the Ānanda that is benign; immoderate eating leads straight to the state of disease that is the terrible.
Food is not the cause of such conditions! Man's delusions, his urges of desire, the cast of his mind: these and nothing else are their foundation. The mind is like a knife with a keen edge. Holding it in your hand, you can cut fruit and eat; or, in a state of fury, you can do harm to life itself. In a good man's hand it is an instrument that does good! In a murderer's hand it turns into a weapon that takes life. For men, the mind alone is the cause of bondage and Mokṣa. If the mind pulls towards bondage, it is terrible; if it pulls towards Mokṣa, it is benign.
Since He who dwells within all, the inner Ātma of every being, is in each one's own mind too, it is natural that He should come to bear the names Ghora and Aghora. When happiness comes, it is the benign: when sorrow comes, the terrible! But happiness and sorrow are not separate states. The absence of the one is simply given the name of the other, that is all. The very absence of happiness is sorrow; the very absence of sorrow is happiness! We can never see them apart.