Basically it is saying the server dropped or kicked you. There can be a number of reasons:
1. You have a previous offence with that server
2. You may break the servers rules (ie too higher ping, custom models etc)
3. You could have poor connection and you cant reach server properly, resulting you in being dropped
Best bet is to try a few more servers.
Also found this..
If you are still having a problem with the "client dropped" error then I'd try deleting a file called "clientregistry.blob" from your Steam folder.....when you next start Steam it will be forced to do a quick update of the platform, which is what I'd do before I decided to try a full re-install