The Nova Cannon is a weapon -- no, THE weapon in Battlefleet Gothic, that fires 50 meter in diameter shells at near-light speed. It is noteworthy for the fact that it creates small blast templates. This may not sound all that impressive until you remember that in Battlefleet Gothic the centre of the base represents the size of a multi-km long spaceship. So on this scale, a nova cannon's blast is equivalent to multi-megaton to gigaton explosion. In one shot. Just think about that. That's the sort of Grade A fucking insane "Humanity Fuck Yeah" ridiculously violent scale we're dealing with in Battlefleet Gothic. This is why it is awesome. It allows something like this to exist.
What this mad bastard actually is varies depending on who you ask. Is it a mass driver? A vortex missile? Every damn bit of explosive on a Forge World shoved into one massive crate and launched at the enemy? What about the "nova cannons" that are giant fuckoff laser cannons or fire beams of plasma comparable to a solar flare? Maybe "Nova Cannon" is just a term that people use to refer to big fuck-off guns. Nobody's entirely sure, but one thing is certain. If an enemy ship has a Nova Cannon you stay the hell away from that magnificent beast unless you wish to suffer sudden massive existence failure. I'm not even joking. Just stay the fuck away.
Shells included one that detonates into a massive short-lived miniature sun-cum-cannonball known for wiping out squadrons or escorts, or an implosion shell that creates a small blackhole briefly to drag enemy ships together to be blasted or maneuvered around or just plain crush them together into a ball of metallic waste.
Alternatively you can just spread your fleet. Seriously. Nova-cannon equipped ships are only effective against clustered formations, and generally despite its awesome AOE, the Nova blast is not powerful enough to guarantee one-shotting anything caught in it, unless it's an escort, an interceptor wing or torpedo volley. Additionally they have a looong reload time, and it's even longer if the ship crew is not of the top quality. Hence why Imperial fleet admirals despise Dominator-class cruisers (armed with a Nova Cannon and little else) and always prefer more reliable ships unless fighting 'Nids and Orks, whose ships DO tend to form in tight formations.
Although, spreading a fleet comes with its own problems in both fluff and tabletop. There are many reasons you would want your ships closer together at times, so a novacannon equipped ship can play a waiting game. If it does, attempting to go after it can be turned into a trap by your enemy to slaughter the attackers and force an opening to use the novacannon. It's very presence is, therefore, a strategic tool: The mere presence of one can force squadron and fleet commanders to disperse formations and anyone aboard anything smaller than battlecruisers or grand cruisers has to fight with the knowledge that they can be incinerated at any point, while anyone aboard a battlecruiser, grand cruiser or battleship (or analogously sized vessels) has to be weary of losing shields lest they get torched as well and anyone on bigger things have to worry about losing shields or portions of the vessel.
GUESS WHAT IS BACK! With the videogame Battlefleet Gothic Armada we now have this monster in glorious Mechanicus-sanctioned third millennium graphic engine, get 3 Dominator cruisers and and a Mars battlecruiser to start inflicting havoc and ruin to heretics and xenos alike. It is however, as inaccurate as it says in Lore, and there is a good chance of missing with every single one of your four Nova Cannon shots, due to RNG if nothing else. So you have people who dislike it for that very reason.
Returning again in the sequel to the above mentioned game, the Nova Cannon now comes strapped to the Apocalypse-class battleship, allowing you to spam lance broadsides between Nova shots.
In the original version of the Battlefleet Gothic rules, the range on the Nova Cannon was prefaced with the letter "G" for "Guess" -- the firing player would guess the distance from the firing ship to the target, and then place the template that distance along the firing line (as determined by a tape measure). This system was also used for artillery in Warhammer 40,000. An eagle-eyed player could nail the target every time. In the transition from fourth to fifth edition, artillery was changed from a guess-range system to a hit-or-scatter system, and so when Battlefleet Gothic was revised, the same change was made to Nova Cannon shots.