Greywater is the water that has been lightly used, like the water in your faucets, showers, and dishwasher. It is not toilet water. There could be traces of food or cleaning products in the water. You should be using environmentally safe cleaning products, body and hand soap, dishwasher soap, and laundry soap, anyway. You’ll be able to use your greywater more safely. The idea is to reuse greywater for other uses that don’t require drinkable/potable water.
The easiest use of greywater is to send it outside to water your garden. The next level is to treat the grey water enough so you can use it for non-potable uses, such as flushing toilets and maybe even faucets, dishwashers, and clothes washing. You can technically filter greywater and treat it to drink, but it might not make as much sense.
Reusing greywater reduces your water consumption from the water company and saves on your water bill.