In Death Stranding the term Timefall stands for rain. It’s a type of rain that is filled with Chiralium, which instantly ages anything it touches by decades, including human skin or cargo. That’s why people seek shelter when it rains and wear long clothes to protect their skin.
Timefall rain is so bad, it even ages the heavy metal containers that Sam carries. Once the container has aged from Timefall it becomes contaminated. Dropping the cargo would cause serious damage to it. There are repair sprays though and you can build a Timefall Shelter to wait under the shelter until timefall has passed.
A special trait of Timefall is that it signals the arrival of BTs. The Chiralium in the air attracts supernatural phenomenon – that’s why BTs (Beached Things / Ghosts) start to appear during rain. You can also see Chiralium Crystals growing from the rain, which can be harvested for crafting materials.
During Episode 3 you recruit a weather station and can then see a weather forecast on your map (press on map) where it will rain next. It’s highly advisable to stay out of rainy areas, and thus away from BTs.
Simply Put: Timefall = Rain – it instantly ages anything it touches, wears out your cargo, and makes ghostly enemies appear