A recovery sentence is an encoded version of your private key using normal dictionary words. The KeepKey recovery sentence is made up with 12 mnemonic words. This sentence should be kept secret. Anyone knowing it can easily get access to your bitcoins.
When initializing your KeepKey device, you will have an opportunity to write down your recovery sentence as a backup to recover your private keys. You will want to keep this copy of the recovery sentence in a safe place.