It allows to access to the issue changes history for the issue tracked by JIRA. You have to know the name used by JIRA to track the attribute (FIELD column value). Unfortunately those field names are documented nowhere as they belong to the private JIRA Java API.  I.e: for the issue status attribute, the field name is: 'status'. You have to figure out the name to use it in the query.

Example: How to get re-assigned issues in DEMO project?

select 
    c.issueid, 
    c.username, 
    c.created, 
    usernamekey(c.fromValue) as "From user", 
    usernamekey(c.toValue) as "To user" 
from 
    jql 
      join 
    issuechanges c on c.issueid = jql.issueid 
where 
    jql.query = 'project = DEMO ' 
       and 
    c.field = 'assignee'