Solow-Swan production function- What is '$L$'
$L$ is labor, which is equal to population in Solow-Swan model. Solow did not actually believe the workforce and population had the same size, but what's important is that labor is supposed to be in direct relationship with population i.e. there is a number w between 0 and 1 such that

$L = w *$ population

Direct proportionality between population and workforce is one of the assumptions of the Solow model.