When you Import a structure, you get a set of issues which should be in the structure and/or information on how they should be arranged in a hierarchical list. Then this information is applied to an existing structure.
For example, you can use a Filter Synchronizer to add All Open Issues to a structure (or the results of whatever Saved Filter you have), or JIRA Agile (GreenHopper) Synchronizer to rearrange issues in the structure according to their rank and epic in JIRA Agile.
To run Import, you must have Control permissions on the structure and additional permissions may be required by a specific synchronizer.
To import hierarchy into a structure:
Select the structure you'd like to import into and click Import link.
If you don't see Import link in the Operations column, then you possibly don't have Control permissions for this structure.
Select a synchronizer from the drop-down list and proceed to configure import parameters.
If there are no synchronizers in the drop-down list, then either none are currently installed or none of the installed synchronizers support import into a structure.
When import is run, it runs under your user name and with your permissions. So if you don't have enough permissions to read values somewhere else or to view issues you'd like to import, you may not see the expected result.