That simply indicates the partition that currently has attention. If you use your mouse to click on the C: partition, it will have those same hash marks. Then you can right-click on it and choose another function, like viewing Properties, for example.
Image
DavidPostillの答えが続きます：
What does it mean when a partition has hash marks on it in Windows 10?
It indicates that the partition has been selected (also indicated, as shown below, by the blue rectangle in the top part of the dialog where a selection is made). Here is an example showing the default selection after opening Disk Management:
Image
After selecting Expansion (F:):
You can see the diagonal lines (hash marks) have moved.
