I find "just right" to be where you have it pulled fairly taut, by hand, but after attaching will still have a slight bit of give. It's not so tight that you can't still stretch or pull it a quarter/half inch by hand without too much effort. If it's a netting with square shaped openings you'd ideally want to see some maintaining their square shape.
Loose is where it's saggy without any extra weight on it. That's probably more likely to tangle wild birds and catch more snow, as the openings aren't nice and open to allow some to fall through.
Netting is kinda hit/miss with snow in general. The most recent snow I got, which was very dry, small flakes, slipped right through the 2" openings with zero accumulation. Past snow which was bigger, stickier flakes, would start building up and eventually need to be knocked off.