After overtaking, the truck behind you flashes briefly once: this is what the driver wants to tell you.

so, the driver is essentially simply saying: thank you.

It is a kind of informal language among  road users . There are various types of these signals. However, they are not all officially defined and can differ from country to country. Therefore, if you see such a signal, you must always assess the situation. After all, a flash of light can mean something else at other times.