This effect/filter fills borders of the input video without changing video stream dimensions. Sometimes video can have garbage at the four edges and you may not want to crop the video input to keep the size a multiple of some number.
The effect has keyframes.
Left / Right - Number of pixels to fill from left / right border
Top / Bottom - Number of pixels to fill from top / bottom border
Mode - Set fill mode. Options are Smear (default; fill pixels using outermost pixels), Mirror (fill pixels using mirroring (half sample symmetric)), and Fixed (fill pixels with Color).
Color - Set color for pixels in Fixed mode. Default is black.
As of this writing in version 23.04 the Color value is ignored.