This query would indicate that 10% or more of pupils on roll are out of age range for the year group they have been assigned. As this is a query if this is correct for your school you are able to annotate this on Collect when uploading your census.
If you do not believe this should be the case I would suggest going to Reporting > Reports > Students (tab) > Student Ages here you can run the report based on the student's age as at 31st August and see if any students look like they are in the incorrect year based on their age.
If any year groups are incorrect these can be amended on the student profile (search the pupil name in the top right hand search bar), once here click onto the extended tab to amend the year and save at the bottom.
In this area it does indicate the year the system would expect the student to be in based on their date of birth - if this seems incorrect please also check the date of birth.