What does LS swap mean?

the car is swapped with one of the LS series motors. Big vette engines i think.
Source - buddy has an LS1 swapped 240sx

It commonly means to remove an older car engine, then replace that engine with a modern GM LS engine. In most older cars, you must replace the tranny too, plus do massive amounts of custom work to make the LS fit a car it was never designed to fit in to.

LS refers to GM's family of V8 engines that first debuted in the 1997 Corvette, not the LS engines of the muscle car era. These engines are everywhere
They can be in the form of $500 junkyard GM truck motors to the LS7 engine out of a Corvette. GM has an annoying habit of reusing engine codes. After the aftermarket jumped on board, parts have become reasonably priced and people started attempting to swap it into just about everything, including imports.