In plain words
We must enquire well into this principle. ‘This is a white cloth. This is an ochre cloth.’ So, though the colours of the cloths appear different, as cloth they are one. The thread that is the basis of the cloth is one. We must take this underlying substance as one. When we grasp that this underlying principle is one, all differences die away. Be it a boy or an old man, all have taken on humanness! Taking up these differences of age and the changing forms of possessions and vehicles, we are spoiling the form. Through the feelings of difference we hold about these names and forms, we fall into many troubles. So we must not take up these forms and their changes. We must take into account the underlying principle.
The underlying substance is one
Śrī Śaṅkarācārya went on teaching this Advaita principle very simply in his Bhaja Govindam ślokas.
Worship Govinda, worship Govinda, worship Govinda, O foolish mind! When the appointed time draws near, no, no, ‘ḍukṛñ karaṇe’ will not save you.
Though the jīva, being born and dying, takes on different bodies, the Ātma within the body is one. That very Ātma shines as the power of Consciousness. As long as it is in the body, the body appears alive with Consciousness. The moment that Consciousness departs, the body becomes inert. Not knowing this, ‘Punarapi jananaṃ punarapi maraṇaṃ’: being born, dying... ‘This alone is our life,’ we think, and we lose heart. If we think about it truly, death and birth belong only to the form; they do not belong to the principle of the Ātma. Once a son who was a Vedāntin was going on with his study of the Veda. By the time the boy had finished studying the Veda, his mother had completed forty years.