It is CRUCIAL to backup your validator's private key. It's the only way to restore your validator in an event of a disaster. The validator private key is a Tendermint Key - a unique key used to sign consensus votes.
To backup everything you need to restore your validator, simply do the following
If you are using the software sign (which is the default signing method of tendermint), your Tendermint Key is located in ~/.secretd/config/priv_validator_key.json.