If you purchase hay - you should know what these numbers mean. It amazes me how many supplements, various and sundry concoctions, - un-natural feedstuffs (kelp, coconut oil, beet pulp, corn oil, etc.) are pushed and people don't even know their basic's on hay analysis.
RFV is relative feed value - a number used by the industry to correlate the total digestible nutrients, protein, acid deterigent fiber, etc into one number that can be used across various hay types and sources.
NDF is neutral detergient fiber - a number that indicates the amount of liginin in the cell wall structure of the plants - or to put it in layman's terms, the stemminess or woodiness of the plant. As in the case of bermudagrass, this is not always as noticable as an overmature timothy.