IRC, which stands for Internet Relay Chat, is a realtime chat facility that many open source projects use to discuss bugs, features, etc. This is useful as developers are often in different parts of the world and different timezones. It is a way for them to communicate and work on solutions in real time.
As a user of a particular piece of software, you may have some questions, or you may simply not be able to get something to work. Using IRC you can chat to the developers, and other users of the software, to troubleshoot, or just generally discuss the software, in realtime.