That's the EFI system partition. Leave it alone. It's normal to have unallocated space here.
That is too big for EFI - it only needs to be 50 Mb in size!
I still recommend 500... You cannot resize it later so easily if needed.
boot - How to know the proper amount of needed disk space for EFI partition - Ask Ubuntu.
In post #2, I should not have used the term "unallocated". I was thinking "unused" file system space. Unused space (in the file system) is normal, unallocated space in a partition is not.
Looking at your image again. You have an unusual situation. From the partition information, the fat32 file system total size is 256 MB (shades of yellow) and the partition is 512 MB leaving 256 MB unallocated (gray). So, how is this explained? Formatting a partition always allocates all the space in the partition for the file system.
Regardless of how this arose, you could still just leave it alone as first suggested, or follow the suggestion in the warning to expand the file system to use all the partition.
I think my bad as thinking of earlier editions of Zorin. Just checked my Devuan install - decided to let it do it's thing - no efi partition here! 
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