Files starting with ._ are created by macOS when copying a file that has extended attributes to a volume that doesn’t support them. This special file is used to save the extended attributes that would otherwise be lost. An example of a volume format that doesn’t support extended attributes is FAT32. These files are hidden when viewing a FAT32-formatted volume in the Finder, but may be shown when you connect it to another device that sees them as separate files.
SyncTime automatically ignores files with the ._ prefix if a file without that prefix exists as well.
If files with the ._ prefix are listed in the sync confirmation window, then it’s probably because the original file without the ._ prefix was deleted but the file with the ._ prefix was preserved. In this case, if you want to avoid syncing files with the ._ prefix, the best solution is to either delete those files manually or add a filter.
If after a sync you notice that the destination contains files with the ._ prefix that were not listed in the sync confirmation window, then probably macOS created them as part of the file copy process. You can confirm this by removing the files again from the destination, then manually copying them in the Finder and checking whether the files with the ._ prefix are created as well.