Recently, Steam has changed their server hosting requirements so that steam-accounts must be linked to an online CSGO server via a token.
This token allows your server to change IPs / ports without affecting the end user.
e.g. If your server IP:PORT changes, the server will be updated on everyone's Server Browser "Favourites Tab".

For you to do create a token, you will need to do a few things in order to comply with these requirements laid out by Valve.

1. Visit this URL: http://steamcommunity.com/dev/managegameservers
2. Enter in the details to the 2 fields being requested
   1. App ID (730 for CSGO, 440 for TF2)
   2. Memo (just a description, "My CSGO Server" is fine)
3. Press the Create Button

Once you have done this, you will see a "Login Token" has been created with the App ID + Memo you have entered in.

Go to our Game Panel, and click on the "manage" button for your server.

Once you are in the "management" page of your server:

1. Click on the settings tab
2. Enter in the "Login Token" where into the "sv_setsteamaccount" field.
3. Tick the "Restart Server" checkbox
4. Press Save

Once you have done these 2 steps, you are now compliant with the new CSGO server requirements laid out by valve.

Keep in mind that if you have multiple servers, you will need to create a new token for each server that you own.