Chinese culture places great importance on auspicious numbers, dates, and words. 吉利的数字 (lucky numbers) include 8 (sounds like 发, 'prosper') and 6 (顺利, 'smooth'). 4 is avoided because it sounds like 死 ('death').
usage
吉利 is colloquial and warm. For more formal contexts, use 吉祥 (jíxiáng). 不吉利 means 'unlucky/inauspicious.'
