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
You can't spoil the porridge with butter.
Repetition is the mother of learning.
Do not chop the branch you sit on
Live for a century, learn for a century, but die being a fool anyway.
Being a guest is good, but being at home is better.
Do not dig a hole for another one, you will fall into it yourself.
Some to the forest, some for firewood.
Get your sleigh ready in summer and your (horse-drawn) cart in winter.