Ok, so what positioning means in general is putting yourself in the best spot at any given moment to do A) Maximize your damage and ability output and B) minimize your exposure to enemy damage and abilities. It's generally referred to in a micro scale, as in a specific team fight, rather than a macro scale. That's just the basic idea though, because good and bad positioning is something thats going to change massively depending on who you're playing. For example:

Good positioning on Nova means finding that sweet spot where you can pop out, burst down the enemy's vulnerable squishies, then disappear again to make it out alive. Bad positioning would be decloaking next to their tank or inside their whole team, etc.

On the other hand good positioning as your team's tank, let's say Muradin, could be putting yourself in a position to both CC enemy squishies to set them up to get blown up by your damage dealers, while also being in position to intercept enemy characters trying to rush your back line in a fight. Bad positioning could be running out of the fight to chase someone when your team needs you to soak damage, not peeling an enemy warrior off your assassin, etc.

So, for Nazeebo, good positioning is going to be the best spot to toss your abilities while staying safe. Keep your beefy muradin/diablo/whoever teammate between you and the enemies. Toss out your spiders and toads then fall back if you dont have someone to cover you while you autoattack the enemy team. I think the biggest fail I see by Nazeebos is trying to use the spirit ulty while standing anywhere near the enemy team. The second that thing pops, people are gonna throw every piece of crowd control they have at you, so if you can have your front line allies between you and that, or cast from behind a wall you're WAY more likely to pump out maximum damage.

If you're using the gargantuan though, you'll probably be playing a little closer in since you wont have to worry as much about not getting interrupted.

Anyway, I hope that made sense in terms of a basic outline of general positioning. Just remember that as Nazeebo everyone wants to kill you always.