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
A fisherman sees a fisherman from afar.
Time for business, an hour for fun.
Get your sleigh ready in summer and your (horse-drawn) cart in winter.
Better to see once than hear a hundred times.
Don't go to the woods if afraid of wolves.
Measure seven times, cut one.
Without labor, you can not even catch a fish in the pond.
Strike the iron while it's hot.