"In the heart of the Sun" - within 17 minutes of arc of the Sun's Ecliptic position and therefore corporally joined to the body of the Sun. The sun's radius on the Celestial Sphere was considered to be 17 minutes of arc. Traditionally a Cazimi planet is fortified.
"is when a Planet is in the heart of the Sun; that is, not distant from him above 17 min. as Mars in 10 deg. 30 min. of Aries, the Sun in 10 deg. 15 min. of Aries; here Mars is in Cazimi." [PA
A planet within 17 minutes of the Sun, by conjunction, is said to be cazimi, and at the "heart of the Sun". This greatly strengthens the planet. It can be likened to a man who has been raised up to sit beside the king.