'A filler is a piece of finished wood used to "fill" openings or gaps between cabinets, or a cabinet and any obstruction, that would inhibit the doors or drawers from fully functioning. ... Typically, fillers are used to finish the space where an overall dimension of cabinetry is less than the overall wall dimension.'

"A filler is used when there is space left over between a cabinet and a wall. There is frequently a little bit of extra space left over, either because a prefabricated cabinet is unlikely to fit a wall's measurements exactly and/or when a measurement is slightly off."

https://www.youtube.com/watch?v=JcMcC55sOIc