Sustainability - huh?

When they say a garment or a label is sustainable - what does this mean? Not gonna lie, about a year ago if you had asked me this question, my response would have been "huh?'' I mean come on, I knew what sustainability was but seriously, who knew it would be a thing and a thing that affected our spend? NEVER 
We all know that our environment is affected by pollution, by our waste dumps, by our fumes however I never thought for one second that clothing would have an impact on our environment