A heavy plasma cannon for eliminating heavily armored targets, like corvettes or bombers. It fires high-temperature plasma blobs that deal massive damage to any vessel. Its drawbacks include relatively low rate of fire and projectile speed, which allow maneuverable targets to avoid hits. However, during Red Corsair's free rein, Imperial pilots preferred these very cannons, reasonably believing that one or two hits would be enough to disable any pirate fighter.
Does it mean "not very sure" or "hope"?
It means that they believed that one or two hits would be sufficient, and adds the author's judgement that their belief was reasonable. It does not necessarily mean that they were correct, only that the author doesn't think they were fools to hold the opinion.
An example from the UK legal system.
The doctor did not to continue treatment reasonably believing that it would be of no benefit to the patient. It was later decided that the doctor was wrong and the patient would have lived if treatment was continued.
When a judge reviewed the actions of the doctor, it was clear that many doctors would have come to the same belief given the information at the time the doctor had access too. Therefore the doctor was not hold responsible for the patients death.