[How-To] Read and understand PHP error messages
 

If you get an error like this:

what does that mean?

It means that some kind of error was in line 125 of (plugin or template)-code executed in somefile line 251.

Now, to track down the error, open somefile and take a look at line 251.
There should be a fetch_hook() or fetch_template()-call in this line.

The identifier tells you the hook or template name that is causing the error.

If it's a hook, there might be multiple plugins attached to it so you have to find out which one causes the trouble by disabling all plugins on this hook and then re-enabling one after one until the error appears again.
Now you got the exact line for the plugin-code that causes the error.

If it's a template, open it and go diretly to the line, most likely you will find an invalid template conditional there.