Fill In Meaning

(transitive) To complete a form or questionnaire with requested information.

Example:   After you're done laying the pipe, fill in the trench.
  If you know anything about this, maybe you can fill me in.
  He can't go on vacation very often because there is nobody to fill in for him.
2011 December 16, Denis Campbell, “Hospital staff 'lack skills to cope with dementia patients'”[1], Guardian:
  The findings emerged from questionnaires filled in by 2,211 staff in 145 wards of 55 hospitals in England and Wales and 105 observations of care of dementia patients. Two-thirds of staff said they had not had enough training to provide proper care, 50% said they had not been trained how to communicate properly with such patients and 54% had not been told how to handle challenging or aggressive behaviour.