plain and simple; naive; unsophisticated
Describes a person, idea, or thing that is straightforward and uncomplicated, sometimes with a nuance of being overly simple or lacking sophistication. Can be used both positively (unpretentious) and negatively (naive).
His way of thinking is plain and simple, so he gets deceived easily.
この絵本は単純素朴な話だが、心に残る。
This picture book has a plain and simple story, but it stays with you.
単純 means 'simple' in a neutral sense, while 単純素朴 adds a nuance of naivety or unsophistication.
素朴 means 'naive' or 'unsophisticated' on its own; 単純素朴 combines both simplicity and naivety.
Compound of 単純 (simple) and 素朴 (naive/unsophisticated). The exact historical derivation is uncertain, but the word is a straightforward combination of its components.