You can recognise this sauce from the brown colour and the visible soya beans in it, it is made by allowing soya beans to decay or ferment. There are many brands, in tall and short bottles, this one is Healthy Boy brand. Once opened keep it in the fridge and it will last many many months. It is used to make fried vegetable dishes and for some sauces.
The main purpose of the sauce in stirfry vegetable dishes is to broaden the flavours.