OpulenZ is an emulation of the Yamaha YM3812, a.k.a. OPL2, FM synth chip which was used i sound cards around 1990 - the Adlib and Sound Blasters. It generates sound through frequency modulation or adding 2 oscillators.
The two oscillators are called operators. In FM mode (bottom left), operator 1 modulates operator 2. Otherwise both operators are routed to the output.
For each operator, the controls are:
ATT, DEC, SUS, REL: an ADSR envelope
KSR: keyboard scaling rate. When KSR is on, envelopes get faster when the pitch rises.
PERC: percussive envelope. Makes the envelope go straight to release ignoring held keys.
TREM: applies tremolo (slight amplitude modulation) to the operator
VIB: applies vibrato (slight pitch modulation) to the operator
FEEDBACK: only for operator 1 - feeds a portion of the operator's output back into ir
LVL: the output level of the operator
SCALE: determines the scaling applied to the output level when the pitch rises
MUL: the frequency multiplier for the operator. 1 stands for the standard pitch played on the keyboard, 0 is one octave below.
WAVEFORM: selection from 4 waveforms: sine, half sine, absolute sine and quarter sine. NB: a DC removal plugin might be a good idea when routing waveforms other than sine to the output.
FM: when this is on, frequency modulation is used. Otherwise the operators are mixed together.
DEPTH, TREM and VIBR: these select deeper tremolo and vibrato. Applies to both operators.
Getting started with FM: http://www.gorenfeld.net/lou/fmguide.html
Programming the AdLib/Sound Blaster FM Music Chips: http://www.shipbrook.net/jeff/sb.html (some of the controls are presented inverted in LMMS, to fit better with other symbols)