It is just like English sayings and phrases, there are Japanese sayings and phrases as well. It is called [kotowaza ことわざ].

Today’s kotowaza

親の心、子知らず　Oya no kokoro ko shirazu

Oya – parents

kokoro – feeling, a heart

ko – children

Children 「do not know how indebted they are [tend to be ungrateful] to their parents. 

Its meaning

Although parents worry lots about their children, children often behaive bad and do whatever they want to do without knowing their parents’ feeling. This means it is half-assed and useless.

An example

My brother did not come back last night. My mother was worried sick. However, my brother came back home next day smiling. It is Oya no kokoro ko shirazu.

arigato