Weapon delivery modes.
The weapons discrete says AIM-9 but the HUD still displays a gun funnel.
Another Guns mode
Probably an Air to Ground - strafing - gun mode... note the altitude bar at lower right.
Detail shots of the various Head Up Display modes in the A-10 Warthog as modelled by Janes Combat Simulations.
These pictures are courtesy of Jan-Albert van Ree