Cameras compress video before sending it — otherwise files would be enormous. H.265 (HEVC) is the successor to the older H.264 and stores the same quality in noticeably less space.
In practice this means a longer archive on the same drives or less load on the network. Both the camera and the recorder must support H.265. Manufacturers often offer their own enhanced versions too (such as "H.265+").
Example
After switching cameras from H.264 to H.265, the same drives keep recordings for longer.