With accent marks:
Запре́тный плод сла́док.
The forbidden fruit is sweet.
Similar proverbs are found in many languages of the world. They all originate from the Bible.
Nowadays, the expression is used as a warning to those who like to enjoy harmful activities: smoking, drinking alcohol, etc.
Other Russian proverbs
An old friend is better than two new ones.
You are greeted by your clothes but bid farewell by your mind.
You can't hide an awl in a bag.
You can't command to the heart.
Bitter truth is better than sweet lies.
Get your sleigh ready in summer and your (horse-drawn) cart in winter.
Patience and labor will grind everything.
Crowded, but not aggrieved.