IRC channels are like individual rooms where conversations take place. Channels always begin with a hash symbol (#) followed by a name which indicates the nature of the conversations and people, in that channel.
Depending on what IRC network you are on there can be hundreds or thousands of channels active, each containing anywhere from one user to hundreds.
Channels can also be private, in that you have to be invited to join them. The majority are public so you will be welcome to join in.