The phrase "significant backlog" means that there is a large amount of work or tasks that have not been completed yet, causing delays or a buildup of work. Full definition
There are significant backlogs in the Irish Probate Office, approximately a 16 - week backlog from the date the application papers are lodged to the date the Grant issues. (lawyer-monthly.com)
As a food blogger, I have a looooong list of recipe ideas and a significant backlog of things I want to make. (mybaconwrappedlife.com)
«Instead he has created a significant backlog of problems across the state. (educationmattersmag.com.au)