The ssh tool stores known remote computer IP address and remote ssh key identification in the ~/.ssh directory.
You can read up on this by consulting the manual page on your mac
man ssh or on the internet.
You may need to read the entire article and do some follow up research if this is your first exposure with ssh - but the relevant section for the warning you pasted goes thusly:
ssh automatically maintains and checks a database containing identification for all hosts it has ever
been used with. Host keys are stored in ~/.ssh/known_hosts in the user's home directory. Addition-
ally, the file /etc/ssh_known_hosts is automatically checked for known hosts. Any new hosts are auto-
matically added to the user's file. If a host's identification ever changes, ssh warns about this and
disables password authentication to prevent server spoofing or man-in-the-middle attacks, which could
otherwise be used to circumvent the encryption.
In essence - either the key changed, the host name changed or the IP address changed and ssh is warning you that one or more of these do not match. You can turn off the storing, the warning or clear the cache if you don't care to investigate more. This is a very useful tool for people that ssh into fixed servers and need to know when things like the above change.