Cyberspace is where online communication happens. If you've spent time chatting with friends on the Internet, you've been in cyberspace.
The official definition of cyberspace is fairly technical: "the notional environment in which communication over computer networks occurs." Since it was coined in 1982 by science fiction writer William Gibson, it's become more popular to use cyberspace to mean "online" or "on the Internet," especially when social interactions happen there. Cyberspace is the almost metaphorical place where two people, separately typing on their keyboards, meet and communicate.