Direct X really basic help.

Ok so I haven't been able to hit this book as much as I wanted to ( Final month or so of high school), I've had to do a lot of stuff the last few weeks. I was doing an example of this book. and I stumble upon this

Code:

//move the ball

ptBallPosition.x+=ptBallVelocity.x;

ptBallPosition.y+=ptBallVelocity.y;

//bounds checking

//left side

if(ptBallPosition.x<=0) ptBallVelocity.x=abs(ptBallVelocity.x);

//top side

if(ptBallPosition.y<=0) ptBallVelocity.y=abs(ptBallVelocity.y);

//right side

if(ptBallPosition.x>=(int)dwDisplayWidth-gdicBall.GetWidth()) ptBallVelocity.x=-abs(ptBallVelocity.x);

//bottom side

if(ptBallPosition.y>=(int)dwDisplayHeight-gdicBall.GetHeight()) ptBallVelocity.y=-abs(ptBallVelocity.y);

I am not really sure what's going in on here with the bounds checking this "abs" business I dont' really know what that is, could someone please clarify this for me? Thanks.