The total volume of a cryptocurrency is the dollar value of all the coins traded at exchanges within a day. For example, the total volume of bitcoin today is $48 billion.
Total volume is a good indicator of how valuable a cryptocurrency is or might become in the future. Large changes in the total volume also indicate large events (such as pump and dumps and deleveraging) or whales (large institutions such as hedge funds or rich people) buying/selling. Related terms include market cap.