According to the Virginia Department of Housing and Community Development....

A cash proffer is (a)any money voluntarily proffered in a writing signed by the owner of property subject to rezoning, submitted as part of a rezoning application and accepted by a locality pursuant to the authority granted by Va. Code Ann. Section 15.2-2303 or Section 15.2-2298, or (b) any payment of money made pursuant to a development agreement entered into under authority granted by Va. Code Ann. Section 15.2-2303.1.

In other words, cash paid by a developer to a locality for the ability to rezone a piece of development land.

There are several bills in the General Assembly that would amend Virginia's proffer system by requiring that proffers be limited to impacts directly attributed to a new development.  Apparently, some localities in the more densely populated areas of Virginia (Richmond, Northern Virginia) are using cash proffers for projects unrelated to the new developments.

As per today's Daily News Record article (County Officials Blast Development Proposal), officials in Rockingham County are not in favor of this bill being passed, because it would require that the County would hire an independent body to examine all proposed proffers in the future, which would be an added cost the County, and thus taxpayers.

My read on the matter is that a reform of the proffer system would improve how things work in Richmond and Northern Virginia (making sure that proffers are really only for projects related to the new developments) but would add additional requirements, processes and costs for the rest of the localities throughout Virginia.