A country's total population size, on its own, actually says surprisingly little about its economic capacity or prospects, since a population figure alone doesn't distinguish between people who are actively working or seeking work and people who, for entirely legitimate reasons, aren't participating in the labour market at all, children, retirees, full-time students. The labour force participation rate, the share of the working-age population that's actually employed or actively seeking employment, gives economic demographers a considerably more useful, economically meaningful number to actually work with.
Two countries with the same population can have very different labour forces
Two countries could have identical total populations while having genuinely different economic capacities, if one has a considerably younger age structure with a smaller working-age share of its population, or a lower rate of labour force participation among its working-age adults, than the other. Because total population doesn't distinguish between these very different underlying situations, demographers and economists studying a country's actual economic capacity turn instead to labour force participation, which specifically measures how much of the population is actually contributing to, or actively seeking to contribute to, paid economic production.
Participation rates shift for reasons a total population count can't capture
A country's labour force participation rate can shift meaningfully over time for reasons entirely separate from its total population size changing at all, shifts in the age structure of the population, changes in how many people of working age choose or are able to actually seek paid work, or broader social and policy changes affecting who participates in the formal labour market. Because these shifts happen independently of raw population change, tracking labour force participation specifically gives economic demographers real insight into an economy's actual productive capacity that simply watching total population trends alone would miss entirely.
What we're still unsure about
The basic distinction between total population and labour force participation, and participation's considerably stronger link to actual economic output, are well established, extensively used concepts in economic demography. What remains more genuinely an ongoing area of research and policy debate is exactly what specifically drives labour force participation rates to rise or fall in a given country over time, since participation depends on a genuinely complex mix of factors, childcare availability, education patterns, health, cultural expectations, policy incentives, whose relative importance economists and demographers continue to actively study and debate, rather than there being one single, dominant factor that explains most of the variation in participation across countries and over time.
This sits inside Labour Force & Economic Demographics, one of seven topics in Demographics, one of four domains in Sociology, one of seventeen subjects the app can quiz you on.