The primary column is always the first column in any of your tables. It cannot be deleted, moved or hidden. 

Usually it is meant to give a short and unique representation (thing, task, project name, objects etc) to each of your row. We use the primary field as brief description in other parts of the UI such as the expand record title or link row tags. 

What it should be? 

The primary column is often a text type field. But it also supports other column types including Number, Date (time), Formula, Email, Auto-number & URL. 

What it cannot be? 

The primary column cannot be an Attachment, Checkbox, Collaborators, Single select list, Multi-select list, Created and Updated time column types. 

Where will you use Primary columns? 

Your primary column becomes your row's identity. You will see your primary column when you -  

  1. You link records between two tables. 
  2. Expand record to change data 
  3. Identify the record on Kanban and Calendar views 

.. and many more.