This is the 11127th most frequent Russian word.
горько
"Bitterly" or "bitter" (used literally or figuratively; also shouted at weddings for the couple to kiss).
Here, 'горько' describes the emotional bitterness caused by the truth.
Он заплакал, ведь правда была столь горькой.
He cried because the truth was so bitter.
In this sentence, 'горько' is an exclamation traditionally used at Russian weddings demanding the couple kiss each other, signifying the metaphorical 'bitterness' of undrunk wine.
На свадьбе гости кричали 'Горько!', подбадривая новобрачных.
At the wedding, the guests shouted 'Bitter!' to encourage the newlyweds.
Here, 'горько' is used to describe the literal bitterness of the medicine's taste.