A security key is an authentication device that strengthens account security when used in addition to a password when signing in. Using a security key is better than receiving codes via phone call or text message because these codes can be phished or intercepted. When you use a security key to sign in, the key will check to make sure you are on the official login.gov website.
A security key is usually a piece of physical hardware, like a USB, that you can carry on your keychain.
I have a physical-hardware code generator for two-factor authentication of my World of Warcraft account.
It's not exactly a security key. I don't plug it into the computer. But I do have to push a button on it to generate a special code that lasts only 30 seconds that I then have to enter as well as my password.
Since I mine Ethereum with my graphics card when I'm not playing World of Warcraft, I've also thought of getting a hardware wallet. I haven't done so yet. But it is likely that I will get one at some point.