Tells NickServ that you are really the owner of this nick. Many commands require you to authenticate yourself with this command before you use them. (The password should be the same one you sent with the REGISTER command.)
/NS IDENTIFY [nickname] password
/NS IDENTIFY password
/msg NickServ IDENTIFY MyNickname MySecr3tpassword
/msg NickServ IDENTIFY MySecr3tpassword
Also consider CertFP or SASL Authentication
Most modern IRC clients can automatically identify you to Services, utilizing CertFP (with an SSL Fingerprint) or SASL Authentication at connection.