A popular genre in Japanese media is the Isekai or "another world" genre, in which the main character is brought from our world to a different one. There are many mechanisms by which this happens such as portals or summoning magic, but the most common method by far is reincarnation after death.

Because it's more interesting if the death is sudden and the main character is unprepared, the death is usually in some sort of accident. The most commonly used accident is a car accident, usually with a truck like the one seen in the post here.

Therefore it's become a meme among people who consume Isekai stories that "Truck-kun" will send you to the afterlife.

The truck pictured is an example of truck-kun, an isekai trope where isekai protagonists get killed by a truck to begin their journey to a life in another world.

From TV tropes: https://tvtropes.org/pmwiki/pmwiki.php/Main/RunningOverThePlot

They’re gods of death and the afterlife.

Egypt has Anubis, the god of funerary rights and protector of graves.

Europe has the Grim Reaper, the soul collector.

Greece has Charon, the ferryman who ferries souls across the River Styx

And Japan has the Box Truck who delivers the unfortunate to their rebirth in another world.

Spoiler