What does this mean? You can reroll or make free sustained?

When a hoverer performs a SUSTAINED ATTACK action, extra care must be taken to keep it as stabilized as possible to increase its accuracy. This also makes the hoverer more vulnerable to attacks from other units. After a hoverer performs a SUSTAINED ATTACK action, until the hoverer’s next activation, all other enemy units can target the hoverer with a SUSTAINED ATTACK action of their own. An enemy unit can only perform this SUSTAINED ATTACK as part of its own activation, and must be within range of the hoverer and use a weapon that can target its armor value to do so

That seems to be a quote from the so-called Advanced Aircraft Rules.

I strongly suggest that you just take those and toss them in trash where they belong.

However, if you intend to persist, I believe that the rule you quote refers to the fact that, normally, when a ground unit attacks an aircraft, it must perform a Sustained Attack and doesn't get to re-roll misses. When a hoverer performs a Sustained Attack of its own, however, a ground unit can perform a Sustained Attack against the hoverer and re-roll it's misses as if it weren't attacking an aircraft.

At least, that's how I interpret it. But those rules are quite poorly written.