This is a very useful feature to bring pull out an older ticket or the exact ticket details without having to apply all the filters to get to one ticket.
The search icon is on the top right-hand corner. 
You can search by Ticket#, Contact Name/email, Tags or the content of the email.
For example, I searched with “Test” and below are the results that pulled out.
You can sort the results by relevance, Last modified and Date created.
You can also categorize your search results by ticket info or the contact info.