After making a serious mistake at work, he knew he had to face the music with his boss.

face 的意思是"面对"，指直面某种情况或人。 the 是定冠词，在这里特指某种特定的情况。 music 的本义是"音乐"，在这个习语中比喻"后果、批评"。 face the music 是一个固定习语，意思是"承担后果、接受批评"，源于军队中犯错士兵被军乐队奏乐驱逐的典故。