The modular synthesizer is a type of synthesizer consisting of separate specialized modules. The modules are typically connected together with patch cords in different ways as needed to generate needed tones. Typical modules are oscillators (operate on frequency), filters (spectrum), amplifiers/gates (amplitude) and Envelope generators (dynamic control). The earliest commercial modular synthesizers were developed, in parallel, by R.A. Moog Co., and Buchla in 1963. Hardware offerings range from complete systems in cases to kits for hobbyist DIY constructors. For more information read the introduction by Roland and How to build a modular synth.
Here is a video that introduces you to Zupiter:
After interesting introduction you can go directly to https://z.musictools.live/ to start playing and experimenting.
Some interesting synth examples with Zupiter:
Acid synth at https://z.musictools.live/#95
Several sequencers in one project https://z.musictools.live/#478
DTMF synth https://z.musictools.live/#273
Here is a link to my earlier posting on other type of web based synths http://www.epanorama.net/newepa/2015/03/27/make-music-in-your-browser/ that you might also find interesting to look at.