What is the difference between Due date and Implementation Target date in JIRA while working on an issue?

[Nic Brough -Adaptavist-]

That's up to you.  

Due date is a built-in field, but "implementation target date" is a custom field that your admins have added for some reason.  You'll need to look at your process to determine what it means.  In fact, a similar rule applies for "due date" - it's up to you to decide what it means in the context of your processes.