This is the 2637th most frequent Russian word.
успокоить
To calm or pacify.
Here, "успокоить" is used to mean calming someone emotionally, often involving reassurance or comforting words.
Я постарался успокоить своего друга после долгого разговора.
I tried to calm my friend down after a long discussion.
In this sentence, "успокоить" refers to soothing a physical or emotional state of another being, specifically reducing fear or anxiety.
Нам удалось успокоить собаку, которая испугалась грома.
We managed to soothe the dog that got scared of the thunder.
In this context, "успокоить" communicates the alleviation of internal emotional turmoil or unease.