The magnifier is used for zooming the picture in or out.
By clicking into the picture the zoom factor is doubled, and the click position is centered.
You can create a rubber rectangle, too. The zoom factor will be adjusted, so that the rectangle is completely visible.
In order to decrease the zoom factor, you have to hold Shift while clicking, or you have to click with the right mouse button (not available on Mac OS).
The zoom factor can be changed via keyboard with "+" and "-" (Windows) or Command+"+" and Command+"-" (Mac OS).
By double clicking the magnifier icon the zoom factor is set to 100%.