When you enter the chat as a guest, the nickname you choose is not reserved for anyone. After you leave, somebody else can type the same name and come in; people who know you may think it is you. This is one of the oldest issues on IRC, and the solution is just as old: nick registration.
Once you register your nickname, the name is yours. When someone else tries to come in with it, the system warns them, and if they cannot enter the password in time they have to give the name up. On top of that, when you log in with a registered name your channel privileges, the groups you belong to and your settings are remembered.
What registration requires
Only a password. An e-mail address is optional, but we strongly recommend it: if you forget your password, that is the only way to get your name back. If you don't give an e-mail address, there is nothing to be done once you forget it.
After registration
You have to identify yourself every time you log in. In the web chat this happens automatically; if you use another program, you enter your password after connecting.