Speed Density - It's a fueling method in which the ECU calculates the amount of air entering the engine using the current manifold pressure, air temperature, RPM, engine displacement, and a 2d volumetric efficiency table in the ECU.
This is instead of the stock fueling method which uses a mass air flow sensor located in the intake pipe to measure the velocity of the air near the middle of the pipe and extrapolate that information to estimate the total mass flow rate of the air entering the engine.
SD is much more stable and reliable at high air flow rates where traditional MAF setups can encounter turbulence. SD is also immune to air leaks anywhere in the intake tract (at least from a fueling perspective). The downsides are that the VE table will need to be re-tuned for even the most minor of modifications, such as a new intake manifold, TGV deletes, valve job, header install, EWG, etc. SD can also be a little finicky at very low air flow rates, such as idle and very light cruise. Some engine management systems allow you to run a hybrid setup that utilizes the MAF at low flow rates, and transitions to SD at medium/high flow rates. I'm not sure if the accessport has this ability though.