Oн поёт вживую - he's singing live (лайв)
он поёт под фанеру - he's miming (майм), he's lip-synching (синк). To mime is more common in British English, though lip-synch is used too. In American English, only lip sync(h) is used.
There's a useful discussion about it here.
British papers use both terms as here (Source: Daily Telegraph) and here (Source: Guardian).
US papers only use lip-sync(h) as here (Source: Time) and here (Source: Washington Post).