Sea ice volume gives a 3-dimensional view of how much sea ice there is. It takes into account the thickness of the sea ice and is expressed in (thousands of) cubic kilometers. This is in contrast to sea ice area and extent which don't take into account the thickness of the ice.
However, in practice it's hard to get accurate estimates of the thickness of the sea ice. There are model-based approaches such as PIOMAS. Such models are useful but may not always match with the actual thickness of the ice everywhere.
There are also observational-based approaches, such as sea ice thickness data from buoys. But those data are usually sparse and are not necessarily representative of the entire Arctic.
The CryoSat data use an observational-based approach using satellite radar altimetry data to measure the freeboard of the sea ice, i.e., the height of the part of the ice that is above sea level, and calculates the ice thickness from that. But there are still issues with that approach as well, such as interference from the snow layer on the ice, and it also requires modeling and has uncertainty.