If I create a "discount coupon" in Houfy, is it visible to everyone who visits my site, or is it "secret" so I can have it for friends and family

You create a discount code and only give it out to the person you want to have it. It is not visible to the public. It’s only visible on your listing if you add it to your description.

The code stays private - Houfy never shows it to visitors. What visitors can see is that codes exist: as soon as you have one coupon, an Enter coupon code box appears on your listing page and in any booking widget you have embedded elsewhere. Guests who do not have a code simply leave it empty, but some may ask you for one, so it is worth deciding in advance what you will say.