Devices your account has recently been accessed from are listed, and you can review them to make sure you recognize them. If you see a device you don't recognize then possibly your account has been compromised and a bad guy has logged in, but make sure you understand all the device fields before you suspect this. For most people it's normal for the IP Address to change sometimes, even if you're always logging in from the same computer. Also, the Description can change when you upgrade your browser, and each different browser you use from a given computer counts as its own device. Finally, if you erase your browser's data then the next time you use that browser it appears here as a new device.
If you're concerned that someone knows your password you can use the Change Password page to set a new password. For a bit of extra safety you can also use the Log Out page to log out, and then log back in with your new password.