There were a beautiful bride and a handsome bridegroom who were sleeping peacefully with smiles on their faces.

peacefully 是形容词 peaceful 的副词形式，由 peace（和平）加后缀 -ful（充满...的）构成形容词，再加 -ly 变成副词。peacefully 意思是"平静地、安宁地"，形容睡眠状态安稳祥和。