If you are drawing onto the Bitmap you need to specify where you want your drawing to take place.
You do this by giving the distance from the top left of the Bitmap. You need a horizontal offset and a vertical offset (often called the x and y coordinates respectively).
These offsets are measured in numbers of pixels starting at zero.
Together these two coordinates are called a Point.
The point at the top left of a Bitmap is always 0,0 and is often called the Origin.
Often you need to use groups of points to specify what you want drawn and where. Say you want to draw a line. To describe where you want it to be you need to state a start Point and an end Point. Many shapes are described using sets of Points. For instance, a rectangle only needs two Points to describe it (just like the line).
ImageGlue has a Point and a PointF structure. The former is used for integral coordinates and the latter for floating point ones. You can cast between these two types and indeed perform arithmetic operations on them.