I get a PHP error - maximum execution time exceeded. What is this?

This error again tells you exactly what has happened. Basically the script has taken longer to execute than the execution time in place for the server.

Now, if nothing major is going on in your script then it could be you have something like an infinite loop - check carefully. If you expect the script to take a long time to run, but want it to do so, then you will need to up the time on the server.

If you want to set to infinite time, you can add a line at the top of the script:
set_time_limit(0);

Note that depending on settings you may need to do this in your php.ini file if you have access to it.