NickServ is an IRC service that protects nicknames on the network. You can register a nick using NickServ, to prevent others from using it. Registering a nick also has its advantages, such as being able to use all available services (with the exception of those only for opers) and joining channels where only registered (umode +r) users can join.
Those who register nicknames have 270 days of inactive use before the nick registration expires. If that happens, you lose everything associated with your account. That is listings on a channel access list, a HostServ vHost, auto-join (ajoin) settings, etc. If your nick is dropped due to inactivity, it becomes open for anyone else to register and use. Therefore, nicknames are first-come first-serve. Of course, you are free to re-register your nick if it still is available but all previously-stored data was deleted.
All NickServ help can be accessed by typing /msg NickServ help. The help info contains info on how to register a nickname, enable/disable protection(s), etc.