I remember the term "critical mass" being used in another context, a long time ago, in a smoky room in Bristol (the only time I've been, although I did quite like it, despite being whacked on the head by a closing traffic barrier on the Clifton Suspension Bridge, which caused me to drop my ice cream).
Anyway... I believe critical mass has something to do with physics, although even back then, in the Bristol flat, it was used to refer to another thing entirely.
In this instance, it's a big bike ride, once a month, when cyclists just gather at a designated time at a designated place, in cities around the world, and go on a saunter around the streets, reclaiming them, if you like.
I've only been once before (see pics) but it's good fun. In the spirit of things, this time I'm decorating my bike (Delia Deux) and have constructed a special structure on which to tie some sparkly things so they don't get caught in my wheels. Also in the spirit of things, I have bought myself a can of Breaker, a beer I haven't drunk since being a student. Ooh, can't wait!