WACC as opportunity cost

Can some one please explain why WACC is seen as opportunity cost? I am unable to understand this from schweser notes

Hi,

The WACC is essentially representative of the rate expected on equities and debt. Therefore, it represents the amount you could have earned if you had invested your capital in the exact proportions of the capital structure. As a result, it is the opportunity cost for investing your money in the investment project.

Hope that helps,