Okay kid, imagine you're in your bed at night and you start thinking about all sorts of things like your day, your friends, your toys, what you want to do tomorrow, and sometimes even something totally random or silly. That's like a "stream of consciousness".
Similarly, when a writer writes in the "stream of consciousness" narrative mode, they try to put down all the thoughts and feelings that are going through a character's mind at that moment without any filter or structure.
It's like you become the character, hearing their thoughts just as they think them. It can be confusing because there's no order, no separation between thoughts and observations or feelings, but it's also really interesting because you feel like you're getting to know the character from the inside out.
So, to sum it up simply, a "stream of consciousness" is like a river of thoughts and feelings that come all at once, and when a writer uses that in their story, they make it seem like you're inside the character's head, listening to their thoughts without any order or filter.