“Wow! The concert was so amazing!”
“Whatever. You know they’re just lip-syncing.”

It’s hard to dance around on stage and sing at the same time, so many performers pretend to sing while playing a recording of themselves singing. They move their lips in sync to the tape, so this is called lip-syncing. “Sync” comes from “synchronize”, which means “do at the same time”. You may have heard it in “synchronized swimming”.