Cultivation refers to the nourishing of the seeds of bodhi by the continual practice of whatever teachings of the Buddha one has been advised to follow.
"What does it mean to cultivate? Here it refers to cultivating precepts, samadhi, and wisdom and wholesome merit and virtue. To walk the other path means to do evil deeds, to involve oneself in greed, anger, and delusion." (TT 43)
Cultivation is like climbing
a hundred foot pole.
It's easy to slide down,
but hard to climb up.
(EDR VII 152)
Although not Buddhist in origin, the following story from the Mencius provides us with a wonderful image of what cultivation means:
The Man of Song
Among the men of Song there was someone who was sorry that his plants did not grow and pulled them higher. Having returned in a dull hurry, he said to his household, 'Today I am really tired! I have helped the plants grow!' His son went running to see them. The plants had withered.
In the world those who do not help the plants grow are few indeed! Those who neglect the plants, considering helping them to be of no use, are those who do not weed the plants. Those who help them grow are those who pull the plants higher. Their help is not merely not beneficial, it is positively harmful! (Mencius, Bk II, Pt. I, Ch.11)
修行