Meditations is a private notebook kept by the Roman emperor Marcus Aurelius during roughly the last decade of his life, much of it spent on military campaign along the Danube frontier. It was written in Greek, in twelve short books, with no title, no structure and no intended reader.
He was the most powerful man in the world when he wrote it, and almost none of it is about power. It is about getting out of bed, tolerating difficult people, doing your work properly, and dying without making a fuss.
The philosophy behind it is Stoicism, absorbed largely from Epictetus — a former slave whose lectures Marcus read obsessively. The central claim is simple and very hard: you control your own judgements and actions, and essentially nothing else, and almost all human misery comes from confusing the two.