How much time you need to complete the task is mentioned as estimated time while logged means how much time you actually spent to complete any Ticket/issue.
Have a look at below image, remaining bar showing the difference between estimated and logged time.
Note: This bar is available on the right side of your ticket window.
How to Add Original Estimate
Step 1: If you want to add the Time Estimate on New ticket then click on create button, after filling all the options, write the estimated time in original time estimate box, same as shown below.
Edit Estimate Time on Existing Issue
If you want to Add/Change the Time Estimate on Existing Issue then click on Edit button like below
Now you will see the ticket box, scroll down the box until you find the Time Estimate Option, change the time and enjoy