It means stores open at least 1 year (and sometimes some change like a month). The company I work for same store sales are up 6.1% Year over Year, while total overall sales are up 24% from new store openings. It’s generally a solid indication of how your established stores are doing.
Most retail store stock prices are off 20-40% in the past 12 months. Regardless of same store reports and consumers stated intent to spend during the holiday season, something’s off.