The purpose of the collet draw bar (or in normal cases, draw tube) is to thread onto the collet tightening it onto the lathe spindle and thus securing the stock to be machined. On a normal sized lathe, the draw tube/ bar thing would be a hollow cylinder with a 1.238" - 20 ID thread on on end, and some sort of locking mechanism. Examples of locking mechanisms include lever or pneumatically actuated ones, simple hand wheels (see picture) or more fancy gear driven hand wheels that are closer to the collet circumventing the need for a long tube.
This picture shows an example of what a normal 5C lathe spindle configuration might look like: