If you own an iMac or MacBook that has one of Apple’s Fusion drives, then the new APFS (Apple File System) will not be supported in the macOS High Sierra software update.
The news of this was recently confirmed by Apple and if you have been a Public Beta tester of the software, then you will need to convert your Fusion Drive back to HFS+. This apparently involves some works and details of what need to be done can be found here.
If you are just upgrading to the new macOS High Sierra, then you will not need to do anything. Any flash drives will automatically be converted to the new APFS and the other drives which include hard drives and Fusion drives will not be converted.
Apple has said that this is not available “in the initial release of macOS High Sierra,” this suggests that they could add support for APFS for Fusion Drives at some time in the future. The new macOS High Sierra is coming next week on the 24th of September.