iOS 12.1.1 fixes several bugs and enhances some features. The update is rolling out now via iTunes or through an over-the-air update on iOS. The update isn't too big; on an iPhone XS it's just 381.7MB.
Notably, the update adds eSIM support for more carriers for iPhone XR, XS, and XS Max users. While the iPhone XR doesn't have 3D Touch, Apple is adding a popular 3D Touch feature through haptic touch: Notification preview.
Last but certainly not least, FaceTime is updated with an easier way to capture Live Photos. We're still waiting on Apple to move the camera flip button back to an easy spot.
Here's how to update:
Be sure to backup your iOS device either via iCloud or by plugging it into a computer with iTunes. Update or not, it's always a good idea to have your data backed up in some form.
Next, you'll need to download the update. Go to Settings>General>Software Update and it will search for an update. From there, you can click Download and Install. If you're already running iOS 12, it will offer to install 12.1.1 overnight when it might be more convenient. On a computer you'll need to be running the latest version of iTunes and navigate to your plugged-in iOS device. Click Check for Update and it will download, than install the latest update.
From there it will install the iOS 12.1.1 on your device and you can get back to using your iPhone or iPad