eating well; good appetite
Refers to eating with enjoyment and a healthy appetite. Often used in contexts of health, recovery, or describing a hearty meal.
I recovered from my illness and can now eat well again.
彼はいつも快食で、何でもおいしそうに食べる。
He always has a good appetite and eats everything with relish.
good appetite and good bowel movements
good appetite and pleasant sleep; enjoying one's food and sleeping well
Compound of 快 (kai, 'pleasant') and 食 (shoku, 'eating'). The exact historical derivation is uncertain, but the meaning is transparent from the kanji.