It means Disk Management has detected the disk as a dynamic type of disk with a "simple" volume.
Normally hdds are "basic" disks. A dynamic disk is supported by Windows 2000 and XP Pro. If a disk is set as dynamic in these systems, you can create "volumes" (like partitions but they are just not called partitions). And you are not restricted to 4 volumes on a dynamic disk.
"Simple" layout on a dynamic disk simply means that the volume uses disk space only from that hdd itself (and not spanned, striped, mirrored etc with other disks).
If it is a new disk, it should not be detected as a dynamic disk. Don't copy files to that disk yet until you've found out the reason.
Is the disk from a previous machine running one of these OS's?