Slashing is a penalty for inappropriate or malicious behavior. Ordinarily, the official node software will not act maliciously unless intentionally altered. When this happens, the validator in question gets slashed (Note: The network treats validator and delegator tokens equally).
Slashing is not currently enabled on the Casper Mainnet. If a validator behaves poorly on the network, they face eviction from the network and loss of rewards. When slashing is enabled, poor behavior will result in token removal. In this case, you will lose any rewards accrued during the eviction period.