Open mic, or open mike, means open microphone. Any audience member can, after signing up for free with the event manager, stand up on stage to sing, play music, read poetry, give a show, ... There is at least one microphone ; most of the time, it is also possible to plug in instruments. Usually, everyone gets on stage for a dozen minutes, typically to play two or three songs.
The audience, the artists, and the organizers are generally open-minded ; however, it's best to check wether your style fits the event, before you come. Also, an open mic is not karaoke, if you see what I mean ;) You don't have to be perfect, but a certain quality is expected.
I hope this website will allow you guys, artists, and organizers, to discover open mics :)