Marketing Development Funds (MDF) is money that a big company (like NVIDIA) gives to its partners (resellers, etc) to sponsor marketing activities (events, conference booths, things like that).
For example, when I worked at Microsoft, companies like NVIDIA or AMD would provide MDF to defray the costs of the Microsoft booth at conferences like SC.
In my personal life, we get the ISPs who service our building (Monkeybrains and Google Fiber) to give us MDF to offset costs of building-wide social activities.