This is explained in the Installation instructions:

Files integrity check
-----------------------------------

The full XOOPS package is released with a script able to check if all the system files have been correctly uploaded to the server. To use it, follow these instructions:

1. Upload the checksum.php and checksum.md5 files located in the XOOPS package root to your XOOPS server folder (putting them next to mainfile.php).
2. Execute checksum.php with your browser
3. If necessary, re-upload the missing or corrupted system files
4. Remove checksum.php and checksum.md5 from your server