Disk transactions per second are how many input/output operations the disk on your server or Miniserver can perform per second. Normal SATA disks will do about 300 TPS.
With Miniservers that is a dedicated minimum figure, and you can burst beyond it.
Disks in a RAID5 or RAID6 configuration can do more (eg. up to 1,000) thanks to the battery backed cache in the RAID controller.