Eating the red berries is showing somebody that it's safe to
do something.
This phrase derived from how a monkey won't eat the red berries unless they see another monkey eat them.
When somebody is being forced to do something and
don't want to do it, eating the red berries would be
doing the thing for them then telling them it's safe to do.
Child: No!
I don't want to eat asparagus!
Mother: You have to eat them! They are healthy!
Child: They look gross!
Mother: Father, help me out here!
Father: Fine, I guess I'll be eating the red berries.
(Father eats some asparagus)
Father: See? It's healthy. Not the tastiest thing on
the planet, but it's tasty enough to eat.
(Child eats some asparagus)
Child: Oh hey,
it's not that bad!