A Wavetable is a set of multiple different waveforms collected into a single group. Individual waveforms are sequentially numbered which is known as index or Wave Position. You can scroll through the waveforms with a knob that governs this Wave Position to bring a waveform “flipbook” effect, so to speak. This brings sonic timbre changes that are otherwise impossible to be obtained from twisting conventional filter cutoff frequency knob. You can even let the synth scroll the waveforms and hence change the sonic timbre all by itself according to the LFO or OSC Envelope set to modulate the Wave Position.