Relating to the words (lyrics) of a song. It's also used more broadly for writing or speech that is beautiful, emotional, and flows like poetry, even if it's not a song - a "lyrical" description of a sunset. The phrase "wax lyrical about" means to talk about something with a lot of enthusiasm and praise, almost like singing its praises.

歌詞的；（文筆）如詩般優美抒情的；wax lyrical about 大讚特讚、滔滔不絕地稱讚。