He means "BAS f", which is a suite of files (scripts, config, html) which are very useful for putting in any mission to get you started.
The command isServer is true if the script is running on the dedicated MP server or if you are in an SP game. This command was added to replace the old workaround, which was to create a gamelogic in the editor called "server", which would allow you to use the code "local server" in order to find out if the script was being run on the server.
The effect of:
?(!isServer): exit
is to stop the rest of the script being run on clients in MP. What you may not realise is that the rest of the script on't run at all in SP, so that may be your problem; if the rest of that script contains errors, they won't be apparent in SP mode (in SP games, your scripts are running on something that is both a client and a server). Unless you give us some more details about what the script you are writing is attempting to do, possibly giving us a link to it or pasting up some of the code in the forum, we can't really be a lot more helpful than that.
One extra comment (based on the fact that you have the SQS code "?(!isServer): exit" in your script): If you are new to scripting, learn the SQF syntax rather than the SQS one. Although the game happily runs it, SQS is pretty much deprecated and is a lot slower to run and harder to read or write.