Creating a NickServ account is much like creating an account on just about any website. Accounts let you mark your identity on a website or service, and prevents other people from being able to impersonate you or use your identity. Accounts are secured with a password, and typically have things like an email address attached to them, so you can be notified of any important changes. On IRC, an account lets you register things such as channels and, most importantly, the nicknames you use.