By default, the Subject field on an Activity is a mandatory free text field. If you'd like to have pre-defined subjects, however, it is possible to use the Activity Category picklist instead or in addition to the free type field..
1. Selecting an Activity Category will auto-populate the Subject field.
You are able to edit the values in the Category picklist to suit your requirements - see this FAQ on how to edit picklist values: https://opencrm.co.uk/help/?kb=1131126&article=How-can-I-change-the-values-in-a-picklist
2. You can completely dispense with the Subject field altogether.
This is achieved by using the Activity Type field instead. To make this work you need to remove the visibility of the field by going to Settings > Default Organisation Fields Access > Activities and then unticking the Visible tickbox.
Important: You will also need to make the Activity Type field mandatory. To do this, go to Settings > Screen Design - Activities and tick the Mandatory tickbox next to the Activity Type field.
3. One more Setting
You need to then tick the following option in Additional Settings under Activity Settings: