Firstly, there is no agreed meaning of what sustainable production is (here lies the cause for confusion and greenwashing). You will find a product can be labelled as sustainable if the brand just has one initiative - using recycled plastic for example. But I would describe sustainable production as producing in a way that does not take more from the earth than it gives back (and could therefore sustain the natural resources used). Not an easy feat! In general terms, a more sustainable product should take into consideration the three pillars of sustainability: ethics, environment and society. Brands tend to lean more heavily onto one of these, for example, Stella McCartney's main message is veganism- so her brand’s initiatives are driven toward ethical and environmental manufacturing.
To get into the nitty gritty, let’s use an organic cotton tee shirt as an example. First, the brand should be ensuring the item is produced in a way that has minimum impact (short supply chains, made in a factory using green energy etc), and is made in a way that is ethical for the whole supply chain of people involved in making that tee shirt. Then, they should counterbalance the impact of the parts that are out of a brands control; farming the organic cotton, processing and spinning the cotton into yarn, dying the fabric etc.
I describe sustainability as a rabbit hole because once you get started there is always more to learn and more to do. This excites and drives me but can be overwhelming for brands trying to create a sustainable product.
Non-sustainable production is where there is no measurement or reduction of environmental impact, no regulations in place to guarantee ethical and safe work, no end-of-life strategy for the pieces being made and no future goals. Typically, producing this way means the brand gets to skip many steps prioritising the bottom line. It’s easier, more available and quicker to produce.