It’s set up to wash laundry by hand.

Edit: since you’re confused as to how that works - water is collected either from rain or brought to this location, maybe there’s running water sometimes, clothes are soaked and scrubbed in the basin and then spread out on the flat stone to dry.

Here’s a link to a more thorough explanation:

https://gringainelsalvador.wordpress.com/2011/04/02/how-to-use-a-washing-sink-el-salvador-como-usar-una-pila/