The Covenant Spirit is a dropship used primarily in battle during Halo 1. It was later replaced by the Phantom, which was larger and more powerful.
The Spirit is a dropship. It has two sides, similar in appearances to a tuning fork with no handle. Each of the "forks" of the ship house Covenant troops, usually Elites, Grunts and Jackals. Upon hovering over the ground, each of the ship's forks will open up along the length of the ship, and the pods inside will allow any Covenant forces inside to jump out and attack.
The Spirit, besides the forces inside it, have one stationary Plasma Turret, similar to the Shade turret, only mounted to the underside of the Spirit. This is a powerful turret, so your two options with the spirit is to either hide until it's dropped it's forces, so you don't get hit up by the cannon. Or, the riskier option is to try to avoid the cannon and fire directly into the ship as it's dropping it's reinforcements to the battlefield.