I received this message with a list of IP addresses from which Steam Guard was being authorised. All of the IP addresses belong to me.
I have previously received a similar message, before being locked out of my account. I had not taken note of the IP addresses, previously, to determine whether they were in fact mine.
Is this anything to be worried about?
The first picture with the list of IPs will show if you have used a new device or new internet browser/app to access steam. If you recognize these IPs then there is nothing to worry about, just make sure to press the button that says "This was me".
The second one will occur if someone accessed your account and you did not authorize it when the popup in picture one shows, or if multiple IPs access your account and just plain old suspicious activity with no authorization from yourself. The only way to get you account back after that is to check your emails, there might be something from steam. If not, you will have to contact steam as stated in the picture.
If the list of IP addresses only list addresses that you use, you have nothing to be worried about, in terms of the potential of another user accessing your account. You should not have these issues, providing you verify each computer to your account.
It appears that you might run into other complications if you are simply using a dynamic IP, and the computer itself does not change. Past that, it is best that you contact Steam support, to determine the best course of action in preventing this sort of thing happening again.
At the end of the day, it does not appear to be anything to worry about in the long run. You have not appeared to be flagged for illegal activity, or anything else that might get your account banned. However, while account limitations like this can be easy to lift, you might worry that should the same problem arise again later, you would have the same inconvenience of lifting it before you can use the features that have been restricted.
