There are two possible meanings: 1) "baptized in water and in the Spirit" or 2) "born physically and spiritually" (See: [[rc://en/ta/man/jit/figs-metaphor]])
The word "kingdom" is a metaphor for the rule of God in one's life. Alternate translation: "experience the rule of God in his life" (See: [[rc://en/ta/man/jit/figs-metaphor]])