wavetable synthesis is a versatile synthesis technique that works by playing back and interpolating between pre-sampled waveforms that are arranged into a "table". One can load many different waveforms into the table and then have a parameter that modulates the position in the table, allowing for smooth timbral variations in sound. The waveforms in the table are all in some base frequency, and then playback speed is increased or decreased to create different output frequencies.
There are many popular wavetable synthesizers out there; the one that's the most well-known from my perspective is Serum. Serum has some iconic wavetables that are widely used in electronic music/dubstep production and it's a mainstay synthesizer.
Waveforms in wavetables are just buffers of samples; any sample or audio source can be used as a waveform. The main requirement is that the base frequencies of all of the waveforms in a wavetable are the same, but you can always correct the base frequencies of individual waveforms if necessary before adding them to the wavetable.
There's a powerful open source GPLv3 licensed wavetable editor available called WaveEdit: https://synthtech.com/waveedit/
It supports loading wavetables, creating new wavetables by either importing and sampling sound files or drawing in the harmonics directly. It has built-in effects that can be applied per-waveform or smoothly across many waveforms. The effects operate in the frequency domain on the FFT buckets.
In addition, it has a share feature where you can browse and fork a collection of very openly-licensed wavetables created by other users. It is/can be used with a variety of hardware and software wavetable synthesizers including the Modbap Osiris Eurorack module.
I suggest using this tool for generating custom wavetables for use with the web synth [wavetable-synthesizer] [module].