This is a set of files and images attached to the step in order to provide some additional information.

Click process step on the canvas (1). Expand "Reference Material" (2).

Click on "add" link (3) to open "New Step File" popup (4):

Select file and click "Upload" button.

Attached files will be shown in the list (5). They can be deleted (6) and downloaded (7):

Attached files will be shown in the "Content" subtab of the proper step in Process Progress Component.