Reference Manual
Orthographic viewports use orthographic camera.
An orthographic camera in 2D viewport views the scene by looking straight down one of the world axes: X, Y, or Z. The Front camera looks straight down the Z axis. The side camera looks straight down the X axis. The Top camera looks straight down the Y axis.
You also have pan and zoom features available in orthographic viewports.
·    Left Mouse Button: Drag to move the camera up, down, left or right.
·    Right Mouse Button: Drag to zoom in or out.
Alternatively, you can push middle mouse button once to activate the orthographic viewports. In order to deactivate the orthographic viewports, push middle mouse button again.