The admin password is what you use to access the HH5's web interface. You can change it from the default to something more memorable but - as you say - no one else should know what it is.
The SSID is what the HH5 sends out to identify itself as a wireless access point. You can change it in the hub's advanced settings - and, indeed, you can have two different names, for the 2.4 GHz and 5 GHz wireless signals respectively, which makes it easier to know what's connecting to what. The name will be picked up by anyone in the area looking for a wireless signal and serves the purpose of identifying which hub it's coming from.
To connect to the wireless signal people need to know the wireless key. Again, you can change this in advanced settings. Your own bits of equipment will need it and you can give it to visitors if you want them to be able to access your wireless. (Or, alternatively, if they're BT broadband customers themselves, they can connect to the BT Wifi signal your hub is broadcasting, without needing to know your wireless key.)
Hope this helps.
The wireless passkey is what you give to people you allow to log onto you homehub to access the Internet. You can change it by following these links
The admin password is what you have to use to gain access to change the settings on the homehub. Only you should have that.
The SSid is the name of your homehub.