Channel operators or "ops" have absolute power over their channel, including the right to decide who gets to come in, who must leave, who may talk, etc. When you first start out, it's best to chat on other people's channels and heed their rules, or else you may find yourself kicked out. If that happens and you cannot settle your differences with the ops, just go to another channel.
At some point you will probably want to try your hand at being a channel op, either by creating your own new channel or by gaining the trust of the ops on an existing channel. You need to know a whole different set of commands. With this power comes the sometimes frustrating responsibility of maintaining the channel against intentional abuse as well as the usual IRC mishaps.