This question was asked live during March 2021 OpenHour, watch the recording here: https://publiclab.org/wiki/openhour-archive
What is Fill Dirt? Fill dirt consists of a mixture of broken down rocks, sand and clay. It contains little fertility for plants to grow, or any organic matter. In building related projects you might hear of fill dirt, in the context of a filler, which provides a more stable foundational material compared to other soils.
What is Topsoil then? Topsoil contains organic matter and is likely to shift or settle over time.