Sprint Estimation is part of the Sprint Turnover process, which happens at the end of the last sprint but before the next sprint starts.
The idea is to check your JIRA for issues that were kicked out of the last sprint, or issues that appeared during the last sprint period. To make this process run smoothly, make sure to tag the issues in JIRA as you go.
For example, any feature-related issues will be planned ahead of time. These issues should be tagged — along with anything kicked out of the previous sprint — with "Issues for Next Sprint". This gives the whole team visibility of what they're going to work on, and helps you play Agile Poker!