In the Walker Assault game mode there are 2 sides: Imperial and Rebel. Imperials are defending the walkers and the rebels are trying to destroy the walkers.

The main objectives for both sides are the two signals. The Imperials want to keep the signals offline and the Rebels want to keep them online. The signals directly correspond to how many rebel Y-wings will attack the walkers and the signal status bar informs you how close the Rebels are to adding a Y-wings to the next attack wave. Ie, the longer the rebels have the signals activated, the more Y-wings will attack at the next checkpoint.

The Imperial side is pretty straight forward to play: kill the rebels, keep the signals offline, and don't let them damage your walker. Imperials win if the walker stays alive and completes the mission at the end of the final checkpoint. Imperials lose if, at any point in the match, the are no remaining walkers.

The rebels can only damage a walker once the shields are down. The dropping of the shields occurs at each checkpoint and coincides with Y-wing strikes. The number of Y-wings that attack is dependent on how successful you were at keeping the signals online. If I'm not mistaken, the walker shields will remain down as long as there are still Y-wings available to attack. Rebels win if they destroy all of the walkers. Rebels lose if the walkers complete the mission at the end of the checkpoint. Rebel speeders can 1-shot a walker using the tow cable but I believe this option is only available for the duration of the final checkpoint.

The top of your HUD provides the details on how close the walkers are to a checkpoint, the number of remaining walkers and the health of each walker, and the number of Y-wings that will attack when the shields go down.