When a truck is passing another truck, quite often the truck being passed will flash the headlights off and back on to "tell" the passing truck that he/she has cleared him, and can bring his/her truck back into the right lane. This is done more often at night but not necessarily only at night. Some drivers flash their high beams instead and this serves basically the same purpose but many drivers do not like this because it can be blinding. Some trucks, though, have headlights that cannot be turned off so there's really no other way.
This courtesy is not really done because the driver doing the passing doesn't know when he's cleared the other truck - it's done as a courtesy - as a way to acknowledge another driver. Some drivers prefer to say something on the CB radio instead, such as, "Okay driver, you got me." Beware! Many states say that this practice is illegal! For that matter, any gesturing to another driver, as in a hand wave for someone to "come around" can make you liable in the case an accident results. Most drivers still wave/flash for other truck drivers, but do not and will not for cars.