This is just a warning mentioning that all vSphere hosts accessing the datastore you are planning to upgrade are vSphere 5 or later. vSphere does this compatibility check and as you can see in the screenshot it confirms that in your case all systems connected are vSphere 5 or later and you can proceed with the upgrade.
If you are planning to share this LUN with a server earlier than vSphere 5, then as mentioned here you will not be able to access the filesystem as VMFS5 is supported only from vSphere 5 onwards. VMFS 3 to VMFS5 upgrade is used very common and the upgrade procedure is very simple.
The main difference between an upgraded VMFS5 datastore and a newly created VMFS5 datastore is that the upgraded VMFS5 datastorewill retain the the original VMFS-3 file block-size and sub-block size.
~Sudhish