QR codes can be used for a lot of things — website links, coupons, tickets, and contact information, just to name a few — but using them with an iPhone or iPad isn't exactly intuitive. Though iOS devices are perfectly capable of reading and acting upon QR codes, that functionality is fairly restricted by default.
The only built-in iOS QR reader functionality is contained within the Wallet app on iPhone and iPod. To access the scanner, open the app, click on the plus button at the top of the "Passes" section, then tap on Scan Code to Add a Pass. From here, you can scan QR codes for coupons, boarding passes, tickets, and the like — but only for the specific things that Wallet considers "passes." If you try to scan any other QR code, you'll get an error message.
Luckily, there are plenty of app developers out there who are happy to add more fully featured QR code-reading capabilities to your iOS device. One of the best is Quick Scan - QR Code Reader, which happens to be free in the App Store. (You can also upgrade to the Pro version of the app for a small sum so you can scan bar codes and access other features.) Simply fire up the app and you'll be able to scan QR codes that redirect you to websites or allow you to access other kinds of data in no time flat. It may not be a built-in app, but it's about as effortless a way to scan QR codes as you can get.