PyLadies PDX has a great resource for getting started on IRC: Getting Started on IRC – PyLadies PDX Portland, OR – Meetup.
IRC (Internet Relay Chat) is one of those tools that’s essential to many open-source (and other!) development communities, but sometimes intimidating and strange to new arrivals. This is a great guide to what it is, how to get there, and some basic commands.
Most large projects end up having a number of different channels for different subjects — for WordPress, the main #wordpress channel covers all kinds of support (from basic how-tos to site/theme/plugin development), and the others tend to stay very focused on specific areas (#wordpress-dev for core development, not site development; #wordpress-ui for UI team work; #wordpress-sfd for support & documentation team meetings). Other projects may break things up on social vs. work lines, skill levels, or work on particular sub-projects.
Work-oriented IRC channels are generally logged (social ones may or may not be). IRC Logs for the official WordPress channels are at irclogs.wordpress.org — look them up by date, time, and channel if you missed a meeting, or when you need to refer back to a discussion in a ticket.