Can explain what this do? I can't make sense of it

Zero condition

if(vy>0) return 1;  // particle is not falling  [b][color=red]{what does return 1 mean?}[/color][/b] [b][color=purple]which i found out later means do nothing[/color][/b]

return y;           // displacement of particle above floor [b][color=red]{why return y?}[/color][/b] [color=purple][b]which i found out later means check for y = 0 to action [/b][/color]

Action      Check box End step at event

// action if particle is falling and below the floor
vy = Math.abs(vy)*coef_of_restitution;  [b][color=blue]{to return the value of Math.abs(vy)*coef_of_restitution to variable vy to simulate hitting n bouncing up like collision}[/color][/b]

ODE [color=blue][b]{Ordinary Differential Equations}[/b][/color] page is

dy/dt = vy


dvy/dt = - g

thx for reply!