Keys are related to the +k channel mode. If someone is an op they can set a key (a password) on a channel with /mode #channel +k keyHERE. You cannot acquire the key without asking an op what it is. Chances are though it's a private channel and you're not meant to be in there.
Invite only (channel mode +i) means you must be invited - only a channel op can invite you, using the /invite command.
Regards,
