Perhaps, this page explains about Jelly well. For me, it's not an event.... it's more like digital life style/ working style.
You can visit the cafe.... bring your computer. Maybe you come with your business associates, partners or simply work there having soft drinks with you. ( OpenSource Cafe requires 500 yen drink to sit and use Wi-Fi. For longer hours, it's recommended to drop more.... right now for 1,000 yen, you can have more than one drink. )
You could find your collaborator there.... at least I can tell that those people around me there have expertise on computer, software etc.
It's so nice to have this kind of place near my place. This is also a good example of my town. There are freelancers, system architects, webby people... as well as artists, creator..... in my area. I love this culture !