"A pony's dreams are inevitably shaped by their mind, and by their experiences. The events of their life are certainly another source of influence, but they obviously remain filtered through the lenses of the pony's own thoughts and inner self," Luna explained.
Rainbow nodded. She was evidently trying to pay attention, but it was clear some level of boredom was there. "I understand, Princess." Carefully, like her entire role as Luna's assistant was at stake, she added, "It does seem somewhat obvious, to me. Not to doubt your knowledge or your teachings or anything."
"I am aware," Luna replied. "This introduction does indeed cover quite obvious matters, but I did think it was nevertheless a necessary one. The rest will hopefully be of more interest to you."
Rainbow Dash nodded again, unsure of whether she was supposed to be embarrassed or not.
"We have been over this before, but it is time we go over it again." Luna cleared her throat, and parted the wall of nebulous darkness beside her with a wing. The space behind was like a starry night sky, but filled with far more stars than Rainbow had ever seen in her life. "Dreams are an expression of a pony's mind, as I was saying."
The space behind the curtain got darker as stars began to disappear, one by one, and Luna continued, "Most of the time, with most of the dreams, this amounts to reflections of their daily thoughts. Hopes, worries, sometimes fears. Love, anger, joy, sadness. On occasion, so warped and twisted they're barely readable, more akin to jumbled nonsense." The space had become fully dark, but in the darkness a shape began to move. Like a snake or a worm, slightly deeper blackness against the backdrop of lightless emptiness.
"But sometimes, the dream latches onto something deeper." There was a shift in Luna's tone, one that took Rainbow's eyes away from the twisting shape and towards her instead. "There are things in a pony's mind, inside every pony's mind, not meant for the outside world. Things not meant to be seen by anyone, save for occasionally a few deeply trusted individuals. The naked essence of a pony's mind. Their desires, their beliefs, everything they allow themselves to be. Ponies are not simple creatures, deep down."
The shape kept convulsing and wrapping around itself, and Rainbow's eyes went back to it. "It is something most ponies don't even realise is there inside them, most of the time. Something that goes unnoticed during their daily lives, almost forgotten. Something even the pony it belongs to only gets in contact with during rare moments. Moments of profound reflection, of great emotional weight, of strong emotions or intense trauma. Moments when a pony truly looks at themself, when the shell of appearances they inhabit is cast off, whether willingly or by force."
The curtain came back, hiding the shape behind it. "I'm sure you have experienced this for yourself, as everyone does." Luna looked straight into Rainbow's eyes. "You must understand, you may come in contact with a dream born of this part of a pony's mind. You might see and know things no one was meant to, things not even the pony themself might fully understand or be aware of. It's very important that you understand the meaning of this."