The ponderings of our overly active mind trying to solve all the problems we took on that day. That is one train of thought I've been told. In dream sleep research they say we all dream or we are suppose to anyway.
I love to dream, in the daytime anyway!
They have meaning if you can get into the habit of writing them down and remembering longer portions of them, then a pattern begins to emerge.
Sometimes, especially in the spring, people have flying dreams, they still tend to scare me awake before I can discover anything, darn it!