This is the 4727th most frequent Catalan word.
regnar
"Regnar" means "to reign" in English.
Here, 'regnar' is used in its literal sense, meaning to rule as a monarch over a region or country.
Durant molts segles, els reis van regnar sobre aquests territoris amb mà de ferro.
For many centuries, the kings reigned over these territories with an iron hand.
In this sentence, 'regnar' is used metaphorically to describe a concept dominating or prevailing in a particular setting, namely peace.
Es diu que la pau regnarà al món quan els conflictes s'acabin.
It is said that peace will reign in the world when conflicts come to an end.
Here 'regnar' is used figuratively to express the pervasive and prevailing presence of a feeling, in this case, joy.