It's all covered in your EOS Rebel T6/EOS 1300D Instruction Manual.

From page 27 we see that it is called the DC Cord Hole.

From page 270 we see than it is a port that allows a wire from a transformer to provide power to the camera via a DC Coupler that is shaped like a battery with contacts in the appropriate places that is inserted into the caemra's battery chamber.

In addition to the fairly expensive Canon branded DC Couplers and transformers, there are also cheaper third party products available that are clones of the Canon units. Just as with third party batteries, use them at your own risk, but if you buy a third party brand with a reputation for quality from a reputable seller, you're probably no more likely to have any issues tan if you buy a genuine Canon unit. Also be aware of the existence of counterfeit Canon-branded products sold by unscrupulous dealers (including third party sellers on amazon - and forget anyone on eBay).

Canon and/or their authorized retailers don't seem to place a priority in keeping them in stock, probably because they're so ridiculously expensive that they don't sell that many. Third party knock-offs are far easier to find on amazon.