What does it mean when a parakeet cackles really loud?

It could mean many things, he may be in distress, or hes just shouting because he feels like it. Most cockatiels, or parrakeet's scream just for the hell of it.

That is the way they communicate. I have 2 parakeets and they do that all day long after uncovering them in the morning. If you haven't noticed, look at the bird and in the front just under the beak is the throat, the bird will pooch it out and sit there and chitter and chatter. A funny thing is that one of my birds snores. When a bird sleeps they will turn their head and tuck their beak under their wing. Well one is quiet while sleeping but the other one has a lower tone chitter while sleeping so all I could figure is that it is snoring. Every bird has its own personality and temperament. I let them fly around inside during the day. At night when I feed them I wait until they go inside to eat then I close up the cage and cover it up. That quiets them up unless they are bickering with each other. If you want a bird, you have to put up with the noise. If it was quiet, it would be sick, so enjoy the noise.

I'm pretty sure It's just the sound they make. I've had several parakeets and they're just loud birds.