In Task Manager, "Disk" refers to the percentage of your hard disk currently being used by your computer to run programs and carry out tasks. It is a metric that you can monitor in Task Manager to see how much of your disks read/write capacity your computer is using to run tasks at a particular time. Disk usage is different from disk storage, which is the total amount of storage space on your hard disk. If you see a high percentage of disk usage in Task Manager, it means that your disk is being overworked and your systems performance may be slowed down.

Disk usage can be reviewed in Task Manager by opening it and clicking on the "Disk" column in the Processes tab. If there are bright orange boxes in the Disk column, that means disk usage is high. Generally, any disk usage that exceeds 1 to 2 megabytes per second (MB/s) on individual processes is suspicious. Its common for your disk usage percentage to periodically spike, but if it remains high for an extended period, it can lead to slow performance speeds, system crashes, and unresponsive systems.

In summary, Disk in Task Manager indicates the disks capacity currently being used by your computer to run programs and carry out tasks. It is a metric that you can monitor to see how much of your disks read/write capacity your computer is using to run tasks at a particular time.