Proverb literally 'a rolling stone gathers no moss.' It means someone who keeps moving or changing will not accumulate possessions, responsibilities, or roots; used metaphorically for instability or constant change.
彼は職を転々としているが、父は「転石苔生さずだ」と心配している。
He keeps changing jobs, and his father worries, saying 'a rolling stone gathers no moss.'
転石苔生さずというが、若いうちはいろいろ経験するのも悪くない。
They say 'a rolling stone gathers no moss,' but when you're young it's not bad to have many experiences.