Overview for obstruction effect by ArmadaMatt. Feel free to download and make a hard copy for personal use.
Rule[]
An attack is obstructed if line of sight is traced through an obstacle token or another ship that is not the defender. If an attack is obstructed by one or more ships or obstacles, the attacker must choose and remove one die from his attack pool before he rolls dice during the Roll Attack Dice step.
- Squadrons do not obstruct attacks.
- If line of sight is drawn over an obstacle, the attack is obstructed even if the bases of the attacker and defender are touching.
Source: RRG 1.5.0 p.13
Line of Sight is obstructed by ships and obstacles.