客is a broad term for a guest, customer, or visitor. In a shop, it refers to a customer; in a home, a guest; and in a vehicle like a taxi, a passenger. It originally referred to someone coming from the outside to visit.
That is how Fuguro works. It brings 客 back for review right before you forget it, each time a little later, until it sticks for good. You hear it spoken aloud, and you meet it again in stories you can already read. The first five levels are free.