A proverb meaning someone with a narrow experience or perspective who doesn't know the larger world (literally: 'a frog in a well knows nothing of the great ocean').
彼は地元だけで成功したからと言って偉いと思っている。まさに井の中の蛙、大海を知らずだ。
He thinks he's great just because he succeeded locally. He's exactly a frog in a well who knows nothing of the great ocean.
海外で暮らしたことのない人にいきなり世界の事情を語られても、井の中の蛙、大海を知らずのように話が噛み合わないことがある。
If someone who's never lived abroad suddenly talks about world affairs, the conversation can fail to connect, like a frog in a well who knows nothing of the great ocean.
教師は生徒に『井の中の蛙、大海を知らず』と言って、もっと視野を広げるよう促した。
The teacher told the student 'a frog in a well knows nothing of the great ocean' to encourage them to broaden their horizons.