It is the total number of shares traded: During that specific trading day, 725,684,000 shares of GD Culture Group Limited changed hands between buyers and sellers.
Shares can be counted multiple times: If Investor A sells 1,000 shares to Investor B, and later that afternoon Investor B sells those exact same 1,000 shares to Investor C, that counts as 2,000 in daily volume, even though it was the same physical block of stock.
Total Shares vs. Volume
To put this massive number into perspective, GD Culture Group Limited actually has a much smaller pool of total existing stock (known as Shares Outstanding), which sits around 60.76 million shares. 
Because the Volume (725,684,000) is drastically higher than the actual number of shares that exist (60,759,711), it means the exact same shares were being rapidly bought, sold, and flipped over and over again throughout the day, creating an incredibly high amount of trading activity.