The Essence
Al-Malik is the absolute King and Sovereign Ruler. Unlike worldly kings whose power is temporary, Al-Malik possesses all things, commands all things, and relies on nothing. He is the ultimate authority.
In the Quran
فَتَعَالَى اللَّهُ الْمَلِكُ الْحَقُّ ۖ لَا إِلَهَ إِلَّا هُوَ رَبُّ الْعَرْشِ الْكَرِيمِ
"So Exalted is Allah, the True King (Al-Malik). There is no deity except Him, Lord of the Noble Throne. (Surah Al-Mu'minun 23:116)"
In Your Duas
اللَّهُمَّ مَالِكَ الْمُلْكِ تُؤْتِي الْمُلْكَ مَنْ تَشَاءُ وَتَنْزِعُ الْمُلْكَ مِمَّنْ تَشَاءُ
"O Allah, Owner of Sovereignty (Al-Malik), You give sovereignty to whom You will and You take sovereignty away from whom You will."
— Quran, Surah Ali 'Imran 3:26