They are slang terms used by people.

Shwagg or Shagg weed is meant to describe very stemmy, commonly brown, seedy, and a very low quality cannabis. In other words, it is shitty weed.

Now Dro, I hate that term. People confuse it with a growing method and slang, but when you see the word Dro' alone, it is usually used to desribe some high quality, good cannabis. Technically though, Dro is shortened from Hydro, which is short for Hydroponics. All that is, is a growing method using no soil, just water. Nothing special about it other than it grows faster, bigger yeilds in a shorter amount of time, and commonly grown using chemical ferts.

Chronic, is again another term used to describe really good cannabis.

All in all, it's just slang.