It is just like English sayings and phrases, there are Japanese sayings and phrases as well. It is called [kotowaza ことわざ].
井の中の蛙大海を知らず I no naka no kawazu taikai o shirazu
I – ito / a well
kawazu – an old way of saying for a frog
Taikai – wide sea
A frog in a well does not know the vast ocean/A frog in a well know nothing of the sea.
A frog in a small well does not know that there is wide sea outside
One knows only things around oneself, but know nothing about the world. (as green as grass, naive) Meaning that one should be aware of the limitations of one’s own experience.
How to use it
One san wa kounai no kaiga konkuru de shoujyou wo moratta. [Shorai wa gaka yo] to sukkari sonoki da.
My old sister received an honorable certificate in a drawing contest at her school. 「I’m gonna be an artist」she is in the mood for it. A frog in a well know nothing of the sea.
We think we know everything but in fact we do not anything at all. There are always much more to learn about. Learning continues until the end of our life, endless. Everyday I learn new things. So I should not over confident myself.