A flexible workforce is one that grows in number to meet needs at any given time and falls back to a baseline number when the increased size is no longer necessary. That capacity is enabled by keeping the number of full-time employees to a minimum and hiring temporary, part-time, freelance and contract workers when required to meet demands.
For the employer, the major benefit of the flexible workforce is lower payroll and other employee-related costs. Another positive aspect is the ability to select talent from a pool of available workers whose skill sets match those required for a given project. The hirer might also be able to afford a high-level expert on a short contract who would be too expensive to have on staff. On the other hand, it can be difficult to access talent with specific expertise when needed, and the business runs the risk of being caught out in emergencies.
From the worker’s perspective, temporary or part-time employment is part and parcel of the ongoing trend to a gig economy. Ideally, the model is powered by independent workers selecting jobs that they're interested in, rather than one in which people are forced into a position where, unable to attain employment, they pick up whatever temporary gigs they can land.
See also: workforce planning