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
Being a guest is good, but being at home is better.
Do not chop the branch you sit on
What goes around comes around.
Each sandpiper praises its swamp.
Laws are not written for fools.
The beauty of a debt is its payment
Patience and labor will grind everything.
Man does not live thanks to bread alone.