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
Do not spit in the well - you will need it when thirsty
If you are chasing two hares at once, you won’t catch a single one.
Beast runs towards the hunter.
Put your trust in God, but don't make mistakes yourself
A holy place is never empty.
There is no bad withouth good.
You can't command to the heart.
It is not a place that makes a person look good, but a person that makes a place look good.