In it's simplest form, non-excessively peppered with financial terminology and equations:
Floorplan is a line of credit dealers use to purchase their retail inventory. Each vehicle on the lot subjects the dealer to finance charges that begin accruing from the moment the dealer takes possession of the vehicle. So, say that the floorplan fees and interest are $100/day per vehicle (for simplicity's sake). The longer a car sits on the lot, the more profit the dealer is losing on that vehicle per day. Make sense?
Why do dealers care?
If you're being charged by the number of days a car is sitting on the lot, and a customer comes in an looks at a brand new, green car that arrived yesterday, and there is an identical twin in every way that is 400 days old, the salesman is going to push you to check out the older one and say they can make a special deal on it for some reason or another (sometimes they'll just say it's because it's been on the lot for a long time if they're really desperate). If you're paying interest on every product by the day, you want to get that out of your inventory as quickly as possible, because that's where you maximize your profit potential.
Often, you can find vehicles on lots that are marked down lower than other similar/identical cars. This is almost always due to floorplan age (I'm assuming we are only talking about new cars here, because used cars bring in a lot of other variables).
Some dealers operate in a way that they seek to capitalize on volume sales vs individual car sales as a way to both be more competitive and mitigate floorplan costs. These are often the best dealers to purchase from (and these are almost never luxury brand dealers) because they will typically sell every single vehicle at invoice price (or several hundred above), and build their profits instead around manufacturer incentives that pay out to the dealer $N per type of car in a graduated fashion where the payout increases as #s of types of cars sold increases. If you have a dealer in your area that advertises (and shows you the proof) that they sell at or near invoice, especially if that is their standard practice, you can safely assume they are a volume dealer. The salespersons are going to be focused on selling as many cars as they can because they are also making their income based on the number of cars sold, not a percentage of per-sale profits.
Can you use this on the lot to your advantage? Sure. Just ask the dealer/salesperson how long the car has been on the lot.