Proper ventilation is critical to keeping indoor air pollutants and moisture out of a home. There are two different types of ventilation: natural ventilation and forced (or mechanical) ventilation.
Natural ventilation, as the name implies, is the process of ventilating a home by natural means, usually by using open windows/doors and other air entry points on a house. Forced ventilation systems utilize mechanical fans and air ducts to cycle air through a house (bathroom fans, attic vents, etc.).
When the inspector comes to inspect your home, they will check for the presence of functioning ventilation in the attic, kitchen, bathrooms, laundry room, and any unfinished crawlspaces, noting any areas that lack necessary ventilation.