256 should denote the position where the platform is per default.
the sin(...) part takes care of moving the platform left and right. Basically it is an expression that "swings" between -1 and 1, see wikipedia (sinus) for details on that.
"time" should be the game time starting from zero on game (or layout?) start. it takes care of the platform actually moving as time progresses, and the "* (360 / 5)" describes that the whole period of movement is split into 360/5=72 steps, each taking place in one time unit.
150 is the distance by which it will move to each direction, meaning it will move 300px in total.
actually as I'm only starting off with construct I would love for someone to confirm I got that right, too ;)
EDIT: and I would also love to see a kind of comprehensive expressions catalogue by object/behaviour type.