When someone has ruminations, they have long and deep thoughts about something.
Ruminations are usually quiet and serious thoughts.
Examples:
Her ruminations about the meaning of life kept her up at night.
His ruminations about the future made him anxious.
I often have ruminations about the stars and the planets.
The cat's ruminations about catching the bird were interrupted by a loud noise.
His ruminations about becoming a superhero were very silly.
The old man's ruminations about the past were full of stories.