What does it mean when it says "server connection timed out"?

If the central server doesn't detect any input from you within a set time period then it will disconnect you so that the "port" becomes available for someone else to use. It prevents someone from logging in and then not doing anything (port hogging).

If you are trying to make inputs but are still getting time out messages then it's possible that either you are being slow with your input actions, or your internet connection is dropping out, or that you have an intermittent fault with your own equipment. If you're logged on at a time when huge numbers of other people are in then the server may simply be overloading and the "inactive" period becomes vary short.

It might be your fault or your internet connection's fault. If it keeps on prompting like that try restarting your computer. If you are 100% sure that your PC is good, then it might be the internet connection. If it didn't got fine after restarting it, try after many hours

it means that the server connection has timed out

Don't worry man. It's the connection. It happens a lot with COD games.