Reefs are large rocks that are very close to the surface of water in the sea. Because sailors cannot see them, they are very dangerous. Ships can easily be destroyed if they hit these rocks. (See: [[rc://en/ta/man/jit/figs-metaphor]])
This is the first metaphor for dead people. Trees in autumn look dead because their leaves have all fallen off, as has the fruit of fruit trees. (See: [[rc://en/ta/man/jit/figs-metaphor]])
This is the second metaphor for dead people. After these "autumn trees" have lost their leaves and fruit, they die their first metaphorical death. Then people uproot them, and they die their second metaphorical death. (See: [[rc://en/ta/man/jit/figs-metaphor]])