What exactly is 'slack'?

Economists like to talk about the "slack" in the labour market. But how can we measure it, and what does it mean? The FT's Brendan Greeley hosts with guests Megan Greene, chief economist at Manulife Asset Management, Ioana Marinescu, economist and professor at the University of Pennsylvania and Mark Blyth, director of the William Rhodes Center for International Economics and Finance at the Watson Institute at Brown University.