不客气 (bú kèqi) is the standard 'you're welcome' in response to 谢谢. 别客气 (bié kèqi) means 'don't be so formal/polite' — used to put someone at ease.
culture
Being told 别客气 is very common in Chinese hospitality. Hosts expect guests to be 客气 (humble, politely declining offers at first), and then hosts insist. It's a social ritual.
