What do hashmarks for a partition shown in diskmgmt.msc mean?

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 marks. Then, you can potentially right-click and choose another function, like viewing Properties.

What does it mean when a partition has hash marks on it in Windows?

It indicates that the partition has been selected (also indicated shown 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:

After selecting "Expansion (F:)":

You can see the diagonal lines have moved.