Subjects are managed on the Settings page, under the subject tab.
Click on the 'new' button, which will display the Add a new subject popup. Enter the name of your subject, pick an icon and colour and click on save.
This subject can now be used within all of your assignments and will organise the files in your Students' OneDrives.
Subjects can be named anything that you want. If you are a sports teacher and you want to share a training brochure to everyone in your team, you may want to have a subject called: Sports training, or Athletics. If you have multiple modules within a subject, you can even use a naming convention e.g. Biology - Photosynthesis, Biology - Internal systems.