Actionscript Vets, help out a noob here.

This is a script i use for character movement(not for a game just for fun).

 Spoiler

Code:

onClipEvent (load) {
    gotoAndStop(1);
    //Variables
    var mod:Number = 7;
    var grav:Number = 0;
    var gravity:Number = 1;
    var fRight:Boolean = true;
    var fLeft:Boolean = false;
    var jmp:Number = -8;
    var ongrd:Boolean = false;
    var grd:MovieClip = grd;
    //End Varibles
}
onClipEvent (enterFrame) {//Start Block
    _y += grav;
    grav += gravity;
    while (_root.grd.hitTest(_x, _y, true)) {
        _y -= gravity;
        grav = 0;
        ongrd = true;
    }
    if (!Key.isDown(Key.LEFT) && !Key.isDown(Key.RIGHT) && !Key.isDown(Key.UP)) {
        gotoAndStop(1);
    }
    if (Key.isDown(Key.LEFT)) {
        gotoAndStop(2);
        if (fLeft == true) {
            _x -= mod;
        } else {
            fRight = false;
            fLeft = true;
            _xscale *= -1;
            _x -= mod;
        }
    }
    if (Key.isDown(Key.RIGHT)) {
        gotoAndStop(2);
        if (fRight == true) {
            _x += mod;
        } else {
            fLeft = false;
            fRight = true;
            _xscale *= -1;
            _x += mod;
        }
    }
    if (Key.isDown(Key.UP) && ongrd == true) {
        grav = jmp;
        gotoAndStop(3);
    }
    if (grd.hitTest(_x, _y, true)) {
        ongrd = true;
    } else {
        ongrd = false;
    }
    if (_root.lw.hitTest(_x, _y, true)) {
        _x = 375;
    }
    if (_root.rw.hitTest(_x, _y, true)) {
        _x = 15;
    }
}

Now the problem i have is with understtanding hitTests. I was hoping someone could break it down and explain them to me.
Heres a sample code i wish tohve explained:
_root.lol.hitTest(_x,_y+10,true)