Change in the marginal utility of leisure with respect to a change in consumption
The partial derivative $\frac{\partial^2 U(\cdot)}{\partial C\partial L}$ tells you how a change in leisure affects the marginal utility of consumption. Therefore if it is positive it means that the marginal utility of consumption is higher when $L=1$. In that case, the agent plans her life to be able to consume more when she is retired and leisure is bigger. In contrast, if leisure reduces the marginal utility for consumption, $\frac{\partial^2 U(\cdot)}{\partial C\partial L}&lt;0$, then it is better to consume more while working, than when leisure is big.