This is the 11079th most frequent Russian word.
гнить
"To rot" or "to decay."
The word 'гнить' here describes the process of wood decomposing due to exposure to damp conditions.
Древесина начала гнить от влажности.
The wood began to rot due to moisture.
The term 'гнить' in this sentence refers to the biological decay occurring within the apple.
Яблоко гниёт с сердцевины.
The apple is rotting from the core.
Here, 'гнить' is applied to vegetables, indicating the likely decomposition caused by inappropriate storage in a warm environment.