The さん on the end is the name honorific さん. It's there to show some respect. It's like saying "Mr. Guest!" or something like that. But that sounds dumb in English. We'll just call this word guest, visitor, or customer. You'll hear this word a lot when you're at a store or you're a customer somewhere in Japan. You are the お客さん, in that situation. Alternatively, you'll also hear お客さま which is a more polite version of this word (because it ends in さま and not さん).
The reading is the reading you learned with the kanji. Just add the hiragana on the end and you'll be good to go.
there is a customer
a guest comes
to be able to see the guest
When a customer comes in, you'll hear the sound of this bell.
I served tea to the customer.
You are one of our precious WaniKani customers.