The origin is the notional (0,0,0) point in the center of the Workspace, the point where all three axis meet. A vector at the origin can be created with
Vector3.new(). The origin is also the CFrame at (0,0,0), usually with a front facing vector of (0,0,1) and a top facing vector of (0,1,0). CFrame.new() will yield this location.
GUI Origin (UDim2)
GUIs have the origin of UDim2.new(0,0,0,0), where the size and positions have scales and offsets of 0 on both axes. This origin is located in the top left corner of the parent element.