what is a "selection rectangle"?

This is a very basic selection option available in almost all programs! Press the mouse in a place where there's no selectable item, drag the mouse while keeping the mouse button pressed, and release the mouse button elsewhere: all the plan items that intersect the rectangle with opposite corners matching the place where you pressed the mouse button and the place where you released it, will be selected.
Pressing the Shift key during this selection operation will toggle the selection state of the items in the rectangle.

what is the "toggle selection" menu?

It's the name of the menu that will be displayed if you press Shift and right click on an object. Try it!