ssh-copy-id is telling me to "checkin" but not telling me "how"

ssh-copy-id is telling me to "checkin" but not telling me "how"

Okay i created a zzz.pub file ( public key ) and then based on google
search , i was to use "ssh-copy-id" to move it to the server and i did. as
seen below. but now it is asking me to "checkin" without giving any
instructions.
[root@oooo ~]# ssh-copy-id -i /zzz.pub IP
root@IP's password:
Now try logging into the machine, with "ssh 'IP'", and check in:
.ssh/authorized_keys
to make sure we haven't added extra keys that you weren't expecting.
[root@oooo ~]#
let's do what it says..
ssh IP
all this did was, ask for the regular server's password.
i don't get it.
let's do it without following it's instructions..
ssh root@IP -i /zzz
okay now it's working fine.
why do they make it sound like it's a "HOTEL TRIP" or something.
can this..
ssh-copy-id -i /zzz.pub IP
be done manually on the server ? must i rely on this command ?
if so , how ?
because.. by default
/root/.ssh/authorized_keys
did not even exist. so i suppose that command created it.
and then i suppose it moved my zzz.pub to there and then CHMODed it to 0400
that's all it did ?
well i did that but it never worked. server was still asking for a
password. ( not a passphrase but server password )