For the last few years, Apple's iMacs have tried to combine the best of both storage worlds in the Fusion Drive, a SSD/hard-drive combo that's meant to blend performance and storage capacity into one. The newly refreshed iMacs are sporting Fusion Drives, but they're much smaller than their predecessors.
The Fusion Drives in previous iMacs combined a 128GB SSD with a harddrive (mostly 1TB). That gave you enough space to store the operating system, and your more important programs and games, helping all that run faster.
In the new iMacs, the 128GB SSD has been reduced to 24GB SSD (in the entry-level 1TB Fusion Drive — the 2TB and 3TB drives still come with a 128GB SSD). That's still big enough to store the OS — meaning you should still get fast boot times — but precious little else. I guess a few corners had to be cut to make that 4K screen affordable.