Hi, Alexandr K
This expression can be translated as 'Simplicity is the truth'. In other words, many fancy things or ideas are not necessarry. 平淡,adj. it means: ordinary, usual, average, nothing special.
Chinese use this expression to show their attitudes towards life. It's about being practical. It can both mean materially and spiritually.
For example:
我们并不需要豪宅,平平淡淡才是真。We don't need a mansion to lead a true life.
结婚之后的生活虽然没有以前浪漫了,但是平平淡淡才是真。
The life after marriage is not so romantic as it was before, but this is what true life is.
Hope it helps.