For a great introduction on synthesis and sound in general, I would recommend taking a look at the web text Music and Computers by Phil Burk, Larry Polansky, douglas repetto, Mary Roberts, and Dan Rockmore. It contains many excellent diagrams and examples.
oscillators and wave tables
Spongefork uses variable sized wave table oscillators. A table is data collection that can be looped. If you loop it fast enough, then you get an oscillator. Faboulous. Since this table size is limited by the amount of memory available to SF, samples loaded into them will need to fit in ram.
FM stands for frequency modulation and is often associated today with cheesy digital synths like the casios. FM means taking the frequency of an oscillator (carrier) and driving it with the frequency of another oscillator (modulator). Spongefork stretches the original purpose of FM to the extreme. At its simplest, with sine waves loaded into the OSC (carrier) and MOD (modulator) you can achieve a harmless vibrato. By increasing the modulation rate (frequency) and depth (amplitude) you can send that sine wave to the moon. Now if you load a sample into one of the oscillators you can achieve very complex modulations due to the amount and varience of both frequency and amplitude in sample data.