Mostly summarized from the textbook 'The Clinical Neurophysiology Primer', by Blume & Rutkove, 2006.
While slowing is considered an abnormality, it's important to remember that it's one of the most common EEG findings. It's not necessarily indicative of seizures, though it is often present in people with temporal lobe focal seizures.
Focal slowing suggests a problem, but doesn't really say what problem. It could be from a tumor, an infarct, or it could be because the person has just come out of a seizure or is having a migraine. (Yes, migraines cause focal slowing.) Even hypoglycemia or hyperglycemia (low or high blood sugar) can sometimes
cause focal slowing.
The biggest thing that is relevant in intermittent slowing is the speed of the wave. The slower it is, the more serious a problem it indicates. If there are continuous bursts of slowing, (It's weird, but that's how they say it. Bursts of slowing merely mean it comes out of the blue, then goes away.) this can suggest eleptogenic focus. (oh, yes, there's the word - it can say the slowing is from epilepsy.)
If the frequency of slowing is rhythmic or shows "frequency evolution," epilepsy is also the most likely reason. Frequency evolution means the slow waves speed up and slow down over and over, but don't speed up to normal during that episode of slowing. Epilepsy is even more likely as the culprit when the focus of the slowing is in the temporal area. (Either of the temporal lobes can be sneaky little brats with this one, the fact that it's a temporal lobe acting up is a major indicator for E.)
So, you might ask, what does slowing look like on an EEG? (Okay, maybe you wouldn't, but I did, so here's a pic.) You can click on either picture to get a larger version.
And here's a normal one: (Can you tell the difference? heh. Good luck.)