It means that the person's output is not simply a multiple of the number of hours that they work, but that for each unit of time they work their output increases compared to the previous unit.

For example:

    Linear
    output = time * k

    Non-Linear
    output = time ^ k

In practice it means that for jobs that are linear, it doesn't matter how many hours any individual works as long as the total hours worked remains the same - assuming that they're all of the same skill level. Doctors, for example, are mostly shift workers so the output of one doctor working for 12hrs is pretty much the same as two doctors working six hours each.

For a job that has non-linear output, two people of the same skill who each work six hours, will be less productive than one person of the that skill working twelve hours. Easy enough to see:

    12 ^ 1.5 = 41.6
    2 * 6 ^ 1.5 = 29.4

In that case, splitting the job between two people results in an output that's 70% of the output of that job being done by one person.

Basically, it's saying that many workers are not fungible assets, no matter that they share the same level of skill.