you learn, without realising it, from what is around you
Proverb; abbreviation of 門前の小僧習わぬ経を読む. Used to express that constant exposure leads to learning, even without formal study.
See also: 門前の小僧習わぬ経を読む (もんぜんのこぞうならわぬきょうをよむ)
By using the computer every day, he picked up programming without realizing it, just like the shop-boy near the temple gate.
Full form of the proverb; 門前の小僧 is the abbreviated version.
From the full proverb 門前の小僧習わぬ経を読む, literally 'a shop-boy near the temple gate recites sutras untaught'. The exact historical origin is uncertain, but it is a well-known Japanese saying.