What this video covers
The clip points at the rubber door seal on a front-loading washer.
The gasket traps water in its folds after every cycle, and the door is usually closed immediately afterwards, so it never dries. Growth builds inside the seal where it is not visible until you pull it back.
The takeaway
Wipe the gasket dry and leave the door ajar between washes. Both take seconds and remove the standing water the growth depends on.
Read the full written guide: Washing Machine Mold.