Hints at the increasing 'casualisation' of the work force, so that while job growth has kept unemployment trending downwards, a significant amount of these jobs are ones where the employee would like to work more (e.g. someone hired to work part-time at 25 hours a week, who would like to work 10 more).
Hope that helps.
"a significant amount of these jobs are ones where the employee would like to work more (e.g. someone hired to work part-time at 25 hours a week, who would like to work 10 more)." = This is defined as underemployment.
To add to duder's response, I would define unemployment and underemployment and state why it has fallen with an example like increase in "gig economy roles" like Uber, Uber Eats, Foodora, etc. You can also mention how people exit the labour force can bring down the unemployment rate as well.
But the response they are looking for I think is that unemployment has decreased and underemployment rate remained fairly steady is because unemployed people taking employment in the "gig economy" and "casual" position thus resulting in a stable rate of underemployment as many individuals would like to enter full-time jobs.