We can measure an object's location in any number of coordinate systems.
In the coordinate system defined by the axes X & Y, the lower-left corner of the triangle is at (0.5, 0.5).
In the coordinate system defined by the axes X' & Y', it's at (0, 0).
A transformation can be thought of as a conversion from one coordinate system to another.
e.g., glTranslatef(0.5, 0.5, 0.0) transforms points from the X', Y' coordinate system to the X, Y coordinate system.
In other words, our program might "start out" using the X, Y
coordinate system.
If the triangle is measured in X', Y' coordinates,
then glTranslate converts it into the
X, Y coordinates for OpenGL to draw.
This view can be useful when creating complex objects out of multiple parts.
Or when performing composite transformations - e.g. to rotate an object about its center.