Yes, that's it. Reshaping is the same as resizing. Reshaping happens the first time the window is opened and any time it is resized.
If you are interested in drilling deeper into when exactly GLUT will call back your function, you might browse the source to freeGLUT.
https://github.com/dcnieho/FreeGLUT/tree/git_master/freeglut/freeglut
Edited to add: Reshaping is also when switching between full screen and windowed.