Literally Wakaf means to stop, contain or to preserve. In shariah, a Wakaf (plural: awqaf) is a sadaqah jariyyah (recurring / ongoing charity), a voluntary charitable endowment, from one's personal belongings or wealth in the form of cash / property for shariah compliant causes.
Its purposes must be:
Once it is dedicated as a Wakaf, the ownership of the asset is transferred to Allah S.W.T. allowing a perpetual use for charity.
You could be a potential Wakif through planned giving; by writing a will (wasiat) to bequeath your asset i.e.from the sale of your HDB flat after your demise.
However, if you have inheritor(s) or heir(s) to your assets, only up to 1/3 of your asset(s) are allowed to be bequeathed as a contribution. This is in accordance to the Islamic ruling of Faraidh.