GOTS is the abbreviation for "Global Organic Textile Standard". The standard sets environmental and social criteria for the processing of textiles and production of garments.
For a fabric or product to be GOTS certified, only natural fibres are allowed to be used that were grown without genetically modified organisms, herbicides, pesticides, synthetic fertilizers. Also, substances that are harmful, toxic and hazardous for people and the environment are prohibited when processing the raw materials (e.g. when dyeing or weaving the organic cotton or organic linen yarns).
Furthermore, social criteria must be met by the suppliers. E.g. Child labour, discrimination are prohibited. Working times, safety and health measurements must be met.
You find more info about the GOTS standard here.