In some forms of Zen training, the student is given a koan. A koan is a question or a story that is puzzling in some way. For example, "What is the sound of one hand clapping?" The discipline is to stay with the koan until you "get" it. Sometimes this takes months, even years.
When the students are monks and live in a monastery, they stay with the koan while they eat, sleep, cook, clean, and also they spend time in intense periods several times a day doing nothing but hanging out with that koan (this is known as zazen, or sitting meditation).
The student stays with the koan intensely, wrestling with it, fighting with it, trying to look at it from different angles, trying to "figure it out," allowing it to be there, and so on. Intensely. They say it's like swallowing the moon which then gets stuck half way down. The frustration can stay at a high pitch for a long time.
And then something happens. The student gets it. Often this is a full-blown "awakening" and the student is never the same again.