Although the passage is not being included, we can assume that the word "placidly" in this excerpt means that an action is being carried out in a relaxed, pleasurable and enjoyable way.
Whenever we do not know the meaning of a word, and we do not have a dictionary at hand, the best strategy to follow is to look at context. This will usually give us a hint or a clue as to the meaning of the word. It can also show us how the word is normally employed.
