AbsolutePosition is a read-only property that provides the screen position of a UI element in pixels. This represents the actual pixel position at which an element renders as a result of its ancestors’ sizes and positions. The GuiObject.AnchorPoint also influences the AbsolutePosition. This property, GuiBase2d.AbsoluteSize and GuiBase2d.AbsoluteRotation are a group of properties that all describe the final rendered orientation of a UI element.
For example, on a 1920 by 1080 screen, a Frame with position {0.5, 0}, {0.5, 0} would have an AbsolutePosition of (960, 540). If you were to place another Frame with position {0, 50}, {0, 50} inside that one, its AbsolutePosition would be (1010, 590). This example assumes each Frame has the defualt GuiObject.AnchorPoint of (0, 0), the top left corner.
This represents the actual pixel position at which an element renders as a result of its ancestors’ sizes and positions. GuiBase2d.AbsolutePosition (roblox.com)
If you have a frame and add another frame as the child , that child’s position and size will be relative to the parent frame’s position and rotation.
You can take the top left corner of the parent frame as the origin(0,0) for the child frame.