What does ob_get_level() > $this->ob_level + 1 mean?

I know it might be a naive question, but I've found this piece of code in the Exceptions.php file and (with all the online documentation) I can't find a way to understand what this means.

PHP Code:

if (ob_get_level() > $this->ob_level + 1)
        {
            ob_end_flush();
        } 


I know it is something about "PHP output buffering mechanism", but it still doesn't make any sense to me.

Any help will be much appreciated.