opencv中一段代码不理解，求解释

[ttcbl]

//加进线段
CvPoint pt1_Rect;
CvPoint pt2_Rect;
pt1_Rect.x = 0;
pt1_Rect.y = 40;
pt2_Rect.x = 384;
pt2_Rect.y = 40;
int thickness = 1;
int ling_type = 8;
CvScalar color = CV_RGB(255,158,97);
cvLine(pFrame,pt1_Rect,pt2_Rect,color,thickness,ling_type,0);

cvShowImage("video",pFrame);
cvShowImage("background",pBkImg);
cvShowImage("foreground",pFrImg);

int x,y;
int a[384][288];
y = 40;
for (x = 0;x < pFrImg->width;x++)
{
a[y][x] = *(pFrImg->imageData + pFrImg->widthStep*y + x);
if (a[y][x] != 0)
{
x = pFrImg->width;
printf("有人进入!");
break;
}
}


这是目标入侵那一段摘录出来的，能否解释下
int a[384][288];
y = 40;
for (x = 0;x < pFrImg->width;x++)
{
a[y][x] = *(pFrImg->imageData + pFrImg->widthStep*y + x);
if (a[y][x] != 0)
{
x = pFrImg->width;
printf("有人进入!");
break;
}
}

中的a[y][x] = *(pFrImg->imageData + pFrImg->widthStep*y + x);
怎样判断有物体进来！